Tonker,
I'm in the same position with a wife and 3 young kids.
Salary during the college is £10,100 plus accomodation allowance, this works out at around 960 quid a month.
You do get asked for your preferences for postings about a month before finishing but these don't mean much. I wrote a 2 paragraph sob story on how I really needed to get posted to Edinburgh and I got posted to Aberdeen
Which has made my life interesting. I'm now needing to get my house sold and move my family 130 miles north. But I can't complain, the job is a fantastic opportunity and I did sign a form saying I was prepared to move anywhere, Aberdeen is at least in the right country
Wages whilst training at a unit are, believe it or not, actually less than what you are on at the college. You get a wage rise but lose the accomodation allowance. I'm currently on 930 quid per month and it should take around 9-12 months for me to validate.