Dave
Because to gain a JAA instructor rating you need to have 300 hours!!!!
Plus you will find employment very difficult in the UK with just a JAA CPL and 150 hours......
When I did my licences I choose to do the FAA system and converted at a later date. But many others do choose the UK route because they feel that would better work for their circumstances. Neither route is wrong, the main thing is to have a plan, a goal for when you have your licences.
Unfortunately what you will find is whatever you budget for just won't be enough.
My plan was to gain my licences in 150 hours but you need that many BEFORE you can take your Commercial checkride, add an instrument, CFI and CFII and you have suddenly added 10-20 more hours. Plus whilst I was in training, there was a regulation change and you suddenly needed 200 hours before you could instruct in a Robinson in the US. Then there is the money for the first job search, which might include a cross US drive for a non-existant job.
Keep asking questions and hopefully Pprune can help save you a few thousand pounds or dollars!!!