Unless anyone else can come up with some more suggestions, the only thing I can think of is searching through local planning permission and scheduled roadworks to see if there is anything planned by BT.
As Blacksheep and others have mentioned several times before, sometimes the issue can be a collapsed duct, or other reason why they cant get fibre to that side of the road. I'll admit that last night I google street viewed that postcode to have a look at the cab to see if I could get any clues. It may just be that there arent sufficient premises from that cab to make the upgrade viable. Looks like Virgin Cable arent there either.
Unfortunately i cant cancel the move just because my internet will be poor.. as much as i'd like to!
You'll have to rate limit what the girls can get, if they are anything like my nieces then streaming will soon take up a chunk of that "Up to 8Mbps" of bandwidth. On the plus side you should be well capable of maximising any internal wiring etc and may even manage to reach the top end estimation of 12.5Mbps.
Depends how much further away from the cab you are I suppose. If you have the current householders phone no, you could try putting it into the adsl checker and it may give you a more accurate result than postcode alone.
Good luck with the move, I believe Mrs_Debug is already well organised and has most of the packing done (including the cat!)