The UK job market for usability-related jobs tends to be very London-centric.
More specialist in User Experience, Usability, UI Design, Information Architecture & UCD roles, a lot of the jobs these agencies carry don't reach the job boards, so send in your CV and get on to their email/call list.
Most recruitment agencies advertise on these job boards. There are others, but if you're using these ones, you're probably seeing most current jobs.
These guys all have jobs posted on their websites. They don't get as many of the roles that are out there, compared with the job boards or the specialist recruitment agencies, but they're still worth a look.