At the very least 2 (the top and the whole/section of the front, for example). Given your location, you might benefit from closing up the enclosure a little, to keep humidity and temepratures more stable regardless of how bad the weather gets.
The top should always be open, obviously, for your lights. But then you have a little more freedom for everything else. You could leave the bottom 1/3 of three sides open (the front and both sides), for example, and as the heat from the lights rises it will pull in fresh air from the bottom. This is how professional vivariums work. Or you could just leave the two sides open completely, and have a solid back and a glass/acrylic front, for viewing.
So you have a bit of wiggle room. If you lived here with me in Florida I would say to have everything as open as possible, due to how hot and humid it is. But England is drier and colder typically, so you can get away with it. It'll depend on your specific home conditions.
Depends, I would say at least two. One on top and maybe the front or side. To look nice Put in three mesh sides. Two on the side and the top portion. Leave the front glass for easy viewing. Really, its fun designing a terrarium.