Hi Ian,
I think you have to ask yourself a lot of questions. Some financial, some personal. Are you motivated? Can you handle heights, have you been on any safety training? Can you clean windows?
I'd try and get a round going on the weekends and build up from there. If you don't want to work weekends between your current job, don't bother even trying. It takes more than you think to be self employed.
However, besides this, you're work on your own, be your own boss, and choose your own holidays(non-paid) and time off to a degree. And also remember rain......there was quite a bit of that in the U.K. when I was there. When it rains, can you do another skill, or do you have something in the pipeline?
Sorry to sound negative, think twice about that old job, at least until you get established. Theres a newbie board here, see what problems they have.