Hi SC, one can still eat well cheaply in London, e.g. their cornish pie is about £2+ and it's big enough to work as brunch. And surprisingly, I find some of their groceries quite cheap (cheaper than in Sg if you're earning pounds), e.g. a large pack of baked waffles or some other pastry like thosai costs £1.80 which can last for 3 meals! These were prices that I saw last Nov.
Stonehenge is good to see, just stand outside the fence and save the exorbitant entrance fee.
You can do the same for those Windsor castle, etc. ...just admire from outside. I like Bath...it's such a quaint town with little houses build close together...it had a very nice Thai restaurant many years ago but I'm not sure if it's still there. Have fun!
ps: sorry, I've no info on how to get to those places, as I'd rented a car and drove out from London many years ago.