An LLU provider will give you the best download speeds - if you can use an LLU provider. So it would be wise to actually check what ADSL services you can get first - before parting with any cash.
Use the link I have put below and it will tell you exactly what ADSL services you can get by just putting in your post code or phone number (i.e. it may say "Sky LLU" or "Be LLU" - that means you can use Sky or Be and they are an LLU service).
Link to ADSL services checker.
You may find you can use an LLU provider such as Sky or Be and then get more bang for your bucks.