I know plenty of people have commented on how easily the insurgents were able to enter the Capitol Building, but I've been thinking about what would have happen if a similar mob tried to enter parliament, and they just wouldn''t get in.
The gates to the Palace of Westminster would have been closed immediately, and any trespassers would immediately have been met by officers armed with Heckler & Koch machine guns, while reinforcements would have been racing in from all over London (and the SAS would probably have been in attendance, even if they didn't get to kill anyone).
You don't invade the legislature like that, you just don't. Protestors sometimes get in and do stupid stuff, but not mobs trying to storm the buildinng.