This is an island of rolling hills and mountains, forests and swaps. All right, that's nice; where is which?
The Pennine Mountains
These form the spine of the Isle, from the north down to the midlands. They stop just noth of where the island gets wide.
The Cambrian Mountains
These cover most of what the Aenglish call Wales; the wide southwestern penninsula. They're very pointy, treacherous, and even worse, filled with Welshmen.
The Pictish Highlands
These form even more craggy and dangerous slopes, but hardly any of the Aenglish will ever have to worry about this, because they are all the way up in northern Alba, and barely anyone even goes as far as southern Alba. Well, except Picts and clergymen trying to convert them. But no, you know, "normal" people.
A fair amount of the island is forested. Actually, a lot of it is forested. Tell you what, let's just move on to the bits that aren't.
Unless you are a low-class fisherman or a peat-gatherer or something, you probably want to avoid the marshy lands. They'll swallow you whole, break horse's legs, and the stench is awful. Plus marsh gas does some really odd things to the unprepared. Expect a lot of mystical visions, put it that way.
The Wash
Much of the land around the bay between East Anglia and Mercia is a big o'l swamp. Go around it.
The Humber
There's quite a marshy area in the basin of the River Humber as well. This is the Vale of York. It's not all swamp; York itself, the capital of Northumbria, is on firm ground. But keep to the roads.
Glastonbury
Along the southern bank of the mouth of the Severn is pretty soggy, and inland along the Parrett River are the wetlands of Glastonbury. This is the location of the Isle of Avilion.
The Thames
The mouth of the Thames River inland a ways is ill-defined. The marshy area comes up to about the old Roman city of Londinium, which is more or less just a fishing village by a ruin these days.
The Coasts of East Anglia, Kent, and Sussex
There are marshy bits here and there along the coast. Get a map before you plan any secret meetings with the French or anything.