The limitations of the poll are vexing. First I'd argue that Rommel and Washington never posed a threat to Britain itself, only its fighting men. Then I'd argue with his micro-managing style Hitler did indeed lead an army against Britain.
Go pre 17th century? Pick a pillager. Although as previously noted assimilation got us where we are. My solidly English ancestors that came here bore a surname that passes through Ireland but has nordic etymology. Go figger.
Also in the Hitler vein, what of him and Goering and the Battle of Britain? Laid far more waste to the mothership than Rommel.
Another vote for Ghandi, he lost the British a jewel in the crown probably more valuable than those pesky American colonies.
And finally, in my war on appeasing peacenik isolationists, I vote for Neville Chamberlain.