I'm not a London UX'er, but I do have experience working with recruiting companies over here in the States. I'm not sure if it's helpful in your neck of the woods, but the best tactic I've found for keeping recruiters honest in their effort to get you into new work is to ask for their supervisor's name and e-mail address, and to include the supervisor on all e-mail communications.
In some agencies, it won't make a difference; hopefully those are ones you can avoid completely. In other agencies, however, keeping the recruiter accountable to their manager in their day-to-day conversations and pitches with you helps to temper some of the difficulties you might otherwise have with going on interviews to positions that are not good matches for your skills and talents. It may seem a bit childish to have to take this step, but in the end, if it helps you to get solid, consistent work, then everyone (agency included) is better off for it.
Good luck to you and your wife!