Hey Shiny - hope you're feeling better now.
Don't start panicking about your dream - it's supposed to be fun! Now, this time I am going to say "Get a grip!" Take a deep breath, have a cup of coffee, and take things a step at a time.
There are loads of places you could live back here. What I suggest is that you get yourself a map and stick the point of a pair of compasses in Perivale, then draw a circle about 60-80 miles radius around it. You can then see what places inside that circle grab you.
If you want any advice on places, I have lived in/been to/stayed in most bits of the UK, so you can have my ever so humble opinion on anywhere, for what it's worth!
Right, now as to the teaching assistant thing. The CRB checks are mainly to check for child abuse. Now I'm not going to pry into what you did, all I will say is that if it was when you were a teenager and there is nothing since, it probably won't make any difference. If you are honest and open about it with a school, and can show that you're a reformed character, they won't hold it against you.
As to the course, the one I did to start off with was a BTec Level 2 Teaching Assistants Certificate with a company called ICS Ltd. You can Google them for the application form eand information etc. They send you a case study school that you can base your assignments on so you don't need an actual placement. The whole course is distance learning - they send you the stuff to read, you post them your assignments, they send you the next load etc. They have forums as well so people can post and help each other. I seem to remember it cost around £200.
Although you will probably have to do an NVQ Level 3 once you are in a job, the BTec course will prove to employers that you really want the job and have been prepared to study off your own back.
Don't give up on your dreams - you deserve to be happy as much as anyone else.