What about the Romans, who taught England both the rule of law and the desirability of highly trained, professional troops?
I think your history is a bit off. The Romano-British culture all but disappeared when the legions were withdrawn in the fifth century. Rule of law dissapeared, and with the Barbarian invasions soon to follow, all ideas of "highly trained, professional troops" were replaced by highly mobile roving bands of pillagers. The only traces of Roman rule at this time were roads and beaten down walls. The Roman church even disappeared for at least a century.
Rule of law can really be said to have reemerged in England in 1066 with the Norman invasions. The Normans brough Norman-Frankish law theory into Britian. During the reign of Henry II (1133-1189) English common law as a concept emerged.
You don't want a history of English common law, I'm sure, but just note that the Roman influence of England, in the grand scheme of history, was minimum.