I have asked around the local airfields and all have given some weird answers from dont bother to it will cost you upwards of £65,000 to obtain the required qualifications.
Well write all them off straight away, obviously ether too lazy to give you the run down or don't know the regulations. Ether way I can guarantee that it will come across in their poor standards of training. Never forget YOU are the customer!
Rant over
In regards to your question. If you are absolutely sure that instructing is all you really want to do (at least for the next 3 years). Then all you really need is:
JAA Class 1 Medical
Commercial Pilots Licence (CPL) Theory
CPL Training and skills test
Instructors Rating training and skills test.
First thing you need to do is get a JAA Class 1 medical examination from the CAA in Gatwick. They are around £400 these days I think.
Next you will need to enrol on a CPL theory course. There aren't that many choices on where to go because most schools only cater for ATPL exams which are pointless if you are 110% sure you only want to instruct and don't want an instrument rating in the next 3 years. They take around 6 months to complete the 9 exams.
As you already have 150 hours, you should be able to go straight in to the CPL course.
After that then its a case of doing the Instructors rating and that's you qualified to teach SE PPL. I think you need a certain amount of instructing experience before you can teach Multi Engine but I could be wrong (and I cant be arsed reading LASORS at 1am)
I would budget for around £15000 - £20000. If you want to do air taxi then you will need to do the ATPL theory and the ME Instrument rating (which is around £15000 - £20000 extra).