While the BHAB Handbook is OK, there are many flying schools that are not members and therefore not listed. If you go to the CAA SRG website, there is a pdf file of FTOs and TRTOs at:
<a href="http://www.srg.caa.co.uk/documents/srg_fcl_ApprovedFTOs.pdf" target="_blank">http://www.srg.caa.co.uk/documents/srg_fcl_ApprovedFTOs.pdf</a>
This should give you a fairly good list of both places to do your instructor rating and/or places to instruct once you have the rating. However, the CAA list is not complete and out of date, so it does not contain all the schools. Try copies of Flyer and Pilot too.
The instructor who does your instructor training is key. Do spend time talking to him/her?; the course is very intense and you will need to get on.
When it comes to getting work, an outfit is likely to look more favourably on you if you have spent some money with them and if they know you.
I was talking to a couple of CFIs recently, and they both mentioned that there was a difficulty getting new instructors.
Good Luck