Have you considered swimming?
It is by far the most single beneficial exercise you can do as it literally uses every muscle group.
But walking 3 miles a day is probably more than most Brits do in a week, so that will be fine.
Whatever you do, do not join a gym. Horrible places full of tarts in lycra, men with sweaty balls and an omnipresent smell of sperm in the corridors. On top of that you will have minimum wage under-qualified staff who more than likely sneak a cheeky ciggy round the back.