I dont think you can maintain historical accuracy and be true to Thomas Mallory. You will have to chose which you would prefer to emphasise. It took over 100 years from when the first germanic tribes landed in the east (Saxons, Angles and Jutes btw) until they reached what could be considered the "welsh" border and they never got a true grip of that western part of teh country. Its in the east and the midlands where most of the real action took place.

Another interesting area is in the north, though, where there was a very strong residual local military presence - some communities survived up there in what had been Roman forts for several hundred years despite the picts and despite the saxons.

There is plenty of academic literature out there on this subject, although I would advise you to stick with anything written after about 1990.