Given that Paris has only one globally significant political target for a light aircraft suicide attack (The Eiffel Tower) whereas London has several (eg Buck Pal, Houses of Parliament, MI6 HQ, London Eye), it does seem odd to me that UK has not adopted an overt, dedicated defence against such a relatively easy attack (far easier than 911) which would nevertheless still have significant political impact.
I expect there are covert defences, but they would only reveal themselves once the political statement has already been deliivered (even if the attack itself fails),