This is what the 1611 translators said exactly in their little understood Preface. I know that it may take a little effort to actually read it properly, but if you do, you will see that the Septuagint did really exist. It was not invented by Origen, but was made in (about) 275 B.C.

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While God would be known only in Jacob, and have his name great in Israel, and in none other place; while the dew lay on Gideons fleece only, and all the earth besides was dry; then for one and the same people, which spake all of them the language of Canaan, that is, Hebrew, one and the same original in Hebrew was sufficient. But when the fulness of time drew near, that the Sun of righteousness, the Son of God, should come into the world, whom God ordained to be a reconciliation through faith in his blood, not of the Jew only, but also of the Greek, yea, of all them that were scattered abroad; then, lo, it pleased the Lord to stir up the spirit of a Greek prince, (Greek for descent and language) even of Ptolemy Philadelph king of Egypt, to procure the translating of the book of God out of Hebrew into Greek. This is the translation of the Seventy interpreters, commonly so called, which prepared the way for our Saviour among the Gentiles by written preaching, as St John Baptist did among the Jews by vocal. For the Grecians, being desirous of learning, were not wont to suffer books of worth to lie moulding in kings libraries, but had many of their servants, ready scribes, to copy them out, and so they were dispersed and made common. Again the Greek tongue was well known and made familiar to most inhabitants in Asia by reason of the conquests that there the Grecians had made, as also by the colonies which thither they had sent. For the same causes also it was well understood in many places of Europe, yea, and of Africk too. Therefore the word of God being set forth in Greek, becometh hereby like a candle set upon a candlestick, which giveth light to all that are in the house; or like a proclamation sounded forth in the market-place, which most men presently take knowledge of; and therefore that language was fittest to contain the Scriptures, both for the first preachers of the Gospel to appeal unto for witness, and for the learners also of those times to make search and trial by. It is certain, that that translation was not so sound and so perfect, but that it needed in many places correction; and who had been so sufficient for this work as the Apostles or apostolick men? Yet it seemed good to the Holy Ghost and to them to take that which they found, (the same being for the greatest part true and sufficient) rather than by making a new, in that new world and green age of the Church, to expose themselves to many exceptions and cavillations, as though they made a translation to serve their own turn, and therefore bearing witness to themselves, their witness not to be regarded. This may be supposed to be some cause, why the translation of the Seventy was allowed to pass for current. Notwithstanding, though it was commended generally, yet it did not fully content the learned, no not of the Jews. For not long after Christ, Aquila fell in hand with a new translation, and after him Theodotion, and after him Symmachus: yea, there was a fifth and a sixth edition, the authors whereof were not known. These with the Seventy made up the Hexapla, and were worthily and to great purpose compiled together by Origen. Howbeit the edition of the Seventy went away with the credit, and therefore not only was placed in the midst by Origen, (for the worth and excellency thereof above the rest, as Epiphanius gathereth) but also was used by the Greek Fathers for the ground and foundation of their commentaries. Yea, Epiphanius abovenamed doth attribute so much unto it, that he holdeth the authors thereof not only for interpreters, but also for prophets in some respect: and Justinian the Emperor, injoining the Jews his subjects to use especially the translation of the Seventy, rendereth this reason thereof, Because they were, as it were, enlightened with prophetical grace. Yet for all that, as the Egyptians are said of the Prophet to be men and not God, and their horses flesh and not spirit: so it is evident, (and St Hierome affirmeth as much) that the Seventy were interpreters, they were not prophets. They did many things well, as learned men; but yet as men they stumbled and fell, one while through oversight, another while through ignorance; yea, sometimes they may be noted to add to the original, and sometimes to take from it: which made the Apostles to leave them many times, when they left the Hebrew, and to deliver the sense thereof according to the truth of the word, as the Spirit gave them utterance. This may suffice touching the Greek translations of the Old Testament.
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The translation of the Seventy dissenteth from the Original in many places, neither doth it come near it for perspicuity, gravity, majesty; yet which of the Apostles did condemn it? Condemn it? Nay, they used it, (as it is apparent, and as St Hierome and most learned men do confess) which they would not have done, nor by their example of using of it so grace and commend it to the Church, if it had been unworthy the appellation and name of the word of God.
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There be many words in the Scriptures which be never found there but once, (having neither brother nor neighbour, as the Hebrews speak) so that we cannot be holpen by conference of places. Again, there be many rare names of certain birds, beasts, and precious stones, &c. concerning which the Hebrews themselves are so divided among themselves for judgment, that they may seem to have defined this or that, rather because they would say something, than because they were sure of that which they said, as St Hierome somewhere saith of the Septuagint.
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This is how the translators of 1611 used the Septuagint: When they came across a hard/rare word in the Hebrew, they could look in the Septuagint and see how it was translated into Greek, so giving them insight into the possible meaning of a word. For example, I read that in Hebrew there is a word "re'em". How did they translated it? The Greek has "monoceros". That is, unicorn. Therefore, I can see how the Septuagint, with all its faults could be used as a secondary witness to the Hebrew, and how correct translations were made into English.

I think some King James Bible people would be shocked to know that this same principle (namely, that there is other witness to the text of the OT besides the Mesoretic Text) was used by the translators of 1611, since they took a holistic view, which included using the Vulgate, Junius' Latin, etc. This would be the honest view. Because they were wise, they could correctly weigh the LXX or the Vulgate, which were not worthless, nor unhelpful.