Unfortunately that is a Market 1 exchange (no LLU), so all he can get is BT / Openreach service
whichever ISP you chose.
PlusNet is almost certainly the cheapest, but the price will be DOUBLE what was quoted above !
The good news is PlusNet is pretty good at customer and tech. support, with call centre in Sheffield.
Do check current BT offerings, see if there is one with no connection fee and 3 months free,
then get PlusNet to match that (they will, their TV ads boast that they won't be beaten on price).
You would get even better deal with phone + BB bundle, as PlusNet is actually owned by BT it will not cause any problems if in future switch back to BT.