As I live on the NE end of Offas Dyke I'd try a Middle Anglo-Saxon 400AP.
Great inspiration! Might be a tricky army to use, particularly against non-historical opponents.
Are There any examples?
One of the guys at our club (Canberra) has been very successful with the Mercian army led by Brilliant Penda, having won every game against historical opponents. The problem is that his list would be significantly different from what you're allowed: he used Wb (S) Hird, Welsh allies and British Cv (O). IIRC the commands were all about the same size, making it easy for him to use the Change Deployment stratagem. To make things a little harder (but more historical for the army) he used only one sub-general, along with a Saxon ally general.
In the list options for Mercia Wb to Sp after 700AD. is this a must?
Must as in a good thing for the army, or must as in compulsory? In the case of the former, yes I think the Irr Sp (O) are all round more useful than the Wb (S). In the case of the latter, yes, the regrade is compulsory.
How would Offa be graded ie. Normal?
Yes, the list is quite specific about who is allowed to be Brilliant or Inert. Only Penda and Alfred can be Brilliant and only Ethelred I and Brihtnoth can be Inert.
Does anyone know names for other leaders of the period?
I got the following men's names from the introduction to the Wikipedia article about Offa, people who were either his close relatives or associates:
Thingfrith
Eowa
Æthelbald
Beornred
Beorhtric of Wessex
Æthelberht of East Anglia
Jaenberht, Archbishop of Canterbury
Ecgfrith
Æthelred of Northumbria
Coenwulf