OK, you need to get a postcode and/or phone number off them so you can check if they can get ADSL. If you get this info, google.co.uk for BT ADSL checker.
If they can get ADSL (and it's hit and miss, they'll mostly find out only after they've moved in), go look at
Demon or
Zen - best two ISPs in the country.
If they can't get ADSL, they may be in a cable area, in which case it depends on the cable operator.
If they can't get ADSL or cable, then they're probably looking at dial-up unless they fancy spending the price of a small car each year on a leased line.
That's just about it. Some areas run wireless trials, but it's a very localised thing.