The reason for all the military choppers flying over North London today is (most likely) to be due to the fact that Regents Park is the home of the US Ambassador to the United Kingdom, somewhere that will host US President Trump on his trip to the UK next week.
The chinooks might be part of a security rehearsal ahead of the visit, something that has been common place for Trump wherever he visits. His arrival next week is widely expected to be greeted with protests.
The BBC reports that the president is expected to bring a fleet of helicopters to the UK – as he did on his last visit. One of them will be Marine One and although it isn’t a specific aircraft, it refers to any US Marine Corps aircraft carrying the president.