There are lots of makes of Freeview boxes, at widely varying prices, but there's very little to choose between them. (The only thing that some people need to look for is the presence of TWO Scart sockets, rather than one. But that only applies if the user intends to connect another device, such as a DVD recorder, to the box. For most users, a single Scart socket is perfectly adequate).
With that minor caveat, simply go for the cheapest box you can find. You definitely don't need to pay more than £20. The cheapest box in Argos is £17.99. A few places sell them for around £15. (The lowest price I've seen was around £12). You might also need to buy a Scart lead. (Don't both with the expensive gold-plated ones in places like Curry's. Any 'pound shop' or 'discount store' will sell you one for around a quid).
All TV signals (both analogue and digital) are broadcast with the intention of providing a decent signal for anyone who has a good-quality outdoor aerial, so there can never be a guarantee that a set-top aerial will be adequate. However the Mendip digital transmitter (which serves Bristol) is now using full power. (It was operating on reduced power until March, in order not to interfere with analogue signals). If your brother could get good analogue reception in the past, he may well be able to get a usable digital signal now. (If not, using an indoor aerial with a built-in amplifier might help).
Chris