...In case you're unaware there were two Dragon Magazine articles written by an acquaintance of mine, Ian Malcolmson, concerning adventuring in Dark Ages England. They are Dark Ages in issue 257 and Hearth & Sword: Deities of the Dark Ages in issue 263. They're by necessity very brief in scope, but nonetheless interesting to read. ...
I've still got my copies of these
Dragon issues just for those articles. They were really well done and seem really good for running a Dark Ages campaign. But, Ambrus is right on how historians would probably debate many things in it. The problem with developing this era as a campaign setting is that it was a very fluid and volatile 500 or so years. Like he said, cultural groups, countries, and borders could and would change from decade to decade. However, there are a few decent maps in the articles showing the major kingdoms at different points, just not a lot of topographical and geographic features.
You'd be surprised. I discovered during my research that a significant stretch of the coastline in south eastern Britain shifted over a mile out to sea since the early Dark Ages. What once started as old Saxon fishing villages and forts are now land locked cities and towns in England.
Absolutely. A perfect example would be the Fens and the Wash in East Anglia. For most of Englands history this was an area of fresh and salt water marsh, much like the Netherlands. The land wasn't reclaimed until fairly modern times. Many of the rivers courses have also changed due to human engineering to drain the Fens. And, along with cities that are now landlocked, there are also cities that have completely disapeared due to coastal erosion.
I lived in East Anglia for about six years and did a lot of reading into the local history of the area. I've always wanted to run an epic campaign spanning the history of the region from Roman times all the way into the Late Medieval period. If you're interested in some cool locations you ought to check out East Anglia. There's Ely (or Isle of Eels), an island in the middle of the Fens with a small port village (Littleport); Bury St. Edmunds, which was once a Saxon royal town and had a very large abbey (not to mention the burial place of King Edmund); Mildenhall, a Danish colony; and Elvedon Forrest (Forrest of Elves) - and that's just scratching the surface.
I'm sure there are some old maps of the area floating around the internet, or possibly from historical societies, that show the area before the Fens were drained. Your best bet for any maps would be to target specific areas you are interested in, then search the internet and contact local historical societies for more info and resources.