I dunno bout england.. but Here we can have turtles..
you would need to have shallow spots and some kind of deep spots I would guess a low of 12 inches and a deep of about 4ft would be good. probably a few feet wide maybe 3-6. and about 3-8 across depending on what all you wanted to keep in it.
you would need a place that they can come out of the water and bask as well.
that would be pretty neat if you ask me! something Id like do do myself!