If going for an AM or FM outfit, 2 channels is the cheapest option, but if going for a 2.4GHz 4 or 5 channel set, there is almost no price difference. Just use the channels you want. You might think of a use for the others later.
There is the advantage of not having to bother about having crystals that don't clash with anybody else. The downside is that there isn't much compatibility between makes, so any future receivers would have to be the same make and possibly model to bind with the transmitter.
Good makes? At the basic level, anything listed by the major UK hobby shops will be OK.