The Broadband Wherever product is also being sold by other firms, some without the extra 25 pounds (the basic cost seems to be 499 plus 200 installation).
I found that there's a list of services from Avanti.
First on the list (Avonline) has a deal where they allow someone to spread the 700 pounds cost into 4 quarterly payments of 175 (for orders placed before 02/04/10). It might be worth contacting them (depending on how serious the likelihood is of moving into the property) and asking if they would allow you to spread the cost, assuming you buy the property - clearly it's unlikely you'd have moved in by their closing date, and would not want to commit to buy before moving in!
Another firm, Prime Satellite Broadband is offering a price as low as 27 +VAT (~31.75) per month, but at 512k down 128k up, using some satellite which goes into service in the next quarter (2010Q2) so it could fit in with plans but the monthly fee depends on length of contract so perhaps tied into 3 years deal...
The products at 34.50/35 pounds seem to have a limit of 2.4 GB so even though the speed would not be an issue, you'd quickly use up the data allowed if you watched much video via the link.
The amount of data is low, however, so it works out much more expensive than any mobile network (unless you go over the allowed limit - Three charges 10p per MB so there's a chance of a bill of over 100 pounds if you exceeded the monthly allowance, though it is only charged by the MB, so you pay for what you use, not some 'chunk' of data which might not be even half used before the end of the month).
If the house is within moderate range of a main road (dual carriageway, perhaps) then thee may be coverage. I'm in N Wales about 10 miles south of Chester, and while there's about 6 km of wire to the nearest BT exchange, and the town centre is an hour's walk away, there are a number of masts serving Three around me (the A483 goes through a cutting, so masts are placed to cover the north and south approaches, and another seems to be for the portion of road between the other two masts) so I have had few connection problems, and have watched video OK.
Hope you find a solution!