That sounds like a good plan. Where would you like to teach? That would be my concern too, think you'd have to do supply for at least a term when you got back so that you'd be around for interviews. I'm more panicking that I won't get a job in the UK for September even being here!
Err... I don't know really! Quite a while I think, in terms of clothes and stuff anyway. I think the bigger you are, the less you notice it to start. If you think, say you start at 18st and lost 1st, which is a lot, that's 5.5% of your body weight, which really isn't that much in the grand scheme of things. Get down to 15st and another 1st off is 6.7% of your body weight so it's a bit more noticeable. Also clothes sizes are strange. Between say a size 22 and 24, it's around 3inches in difference but a 14 down to a 12 is 1-1.25 inches so the bigger you are, the more you have to lose but only to start with! At my biggest I weighed 19st 13lb (well actually I was 21st at one point) and a size 24-26. I'm now 15st 13lb (plus a few lbs at the moment still probably!) and a size 16-18. So 4 stone down and I've dropped 4 sizes. Feels like I've still got a very, very long way to go though!