There's no 'bare ADSL' available, you have to pay line rental on top (partly to cover maintenance of the line in case it needs repairs).
Unfortunately if there are discount schemes (like the old 'Light User Scheme' but I cannot think of the name now - aimed at people eligible for benefits and giving reduced rate line rental, as line is effectively for inbound and emergency calls) then you are barred from having ADSL.
Similarly, there's another 'barred' option (to stop people running up huge debts) which makes the line ineligible for ADSL.
Mobile dongles might be worthwhile (*), but minimum cost is often 10 pounds for 1 GB of traffic (where a landline might be used for 20-40 GB in a month if someone used YouTube or BBC iPlayer or watched TV shows off Channel 4 or Five "on demand". PAYG usually work out more costly than contracts, and there'd probably be a credit check for long term contracts.
Although TalkTalk sometimes gets a bad press, they are doing broadband at 6.49/month (plus their own line rental of 11.50 a month). Might go up with VAT change in new year, and like best mobile deals, there is a contract for 18 or 24 months.
(*) Three was offering 5 GB for 7.50/month in September 2008, but now charge 15 quid for 15 GB (OK, if he has Three coverage, but 18 or 24 month contract).
Finally, there are a couple of "Three" mobile dongle deals which might be worth looking at - would offer longer term access for a one-off fee. 99 quid gets the dongle plus 12 GB of traffic for up to 12 months - expires at the end of that time or when all data used up. You could monitor usage to see if it is going really fast or really slow and then help brother make a better choice for long term access.
Might start slow and build up if he goes to YouTube or other video sites, IYSWIM.