I can't advise on BT Broadband for Business, but advise very strongly against Sky non-LLU, (Sky Connect). Sky LLU is apparently reasonably good, but Sky Connect has a dreadful reputation.
How much download do you use at the moment, and is your upload volume significant? What is your current Newnet allowance and price? Can any large downloads, (you can't be doing large ones on Newnet

!), be scheduled to run overnight?
There is no such thing on a BT Wholesale circuit such as yours as "cheap unlimited broadband with no throttling". However there could be reasonably priced alternatives that give you more for your money. I can think of four possibilities off the top of my head, but need more detail from you as above to advise if any of them really fit.
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Internet connection - IDNet Home Starter Fibre.
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