On that exchange there are only Sky and TalkTalk LLU suppliers, so there is a possibility that all the Sky LLU Unlimited connections have been taken up. I don't know how accurate the checker you have used is.
If you can get on the Sky LLU service that should be OK, but if it is fully taken up then they would put you on Sky Connect, the non-LLU service. That is dreadful. So you need to make sure with their sales that they have space.
Is the cost a prime consideration? If not, there are probably other suitable LLU ISPs selling TalkTalk Business Wholesale if you can't get that Sky one, and the exchange is due for 21CN (WBC) at the end of this month which would open up the door to decent non-LLU.