Are you willing to move away from London? Getting away from somewhere you've already found out is saturated with ex-cabair instructors will be a good start. When I was looking, quite a lot of the vacancies were in the north rather than the south, but you never know what might come up.
I downloaded the list of registered training organisations from the CAA website, and called a fair number of these asking to book an appointment to discuss instrucor vacancies. I found this approach to be very sucessful, and any schools which may have had vacancies coming up were happy for me to come in and chat.
Having an appointment is obviously going to work out better, as you are going to be talking to the right person who has the power to make the decision.
http://www.caa.co.uk/docs/33/srg_lts..._Doc31v105.pdf