As pointed out already, Prince Andrew, Prince Edward, and the Princess Royal (Anne) all have places to crash in London. Anne has some digs at St. James Palace, and office space at BP. I'm sure there's ready accommodations available for any and all royal family members at BP if/when needed. No need for hotel reservations, ahem. The perks of being royal are surely something we can barely imagine! The Sussexes reportedly will continue to maintain offices at KP. For the moment, nothing has been said about whether or not they will retain Nott Cott for a crash pad. In any case, Will & Kate surely have enough room in their 21-room Apt 1-A for an occasional stay-over by any family member.There is not really any reason why the Duke and Duchess of Sussex should have a pied-à-terre in Central London. Frogmore Cottage is about 45 minutes' drive away from Kensington Palace. Prince Andrew and Prince Edward live in the vicinity and don't have second homes in London...
There's a whole lot we don't know about the inner workings of the British royals and their household matters. Still, some things are fairly well known, but not examined that much. Now, with Harry marrying an American divorcee of variant ethnic background, there's been increased interest in the goings-on of the British royal family. Not that everyone gives a flying hoot. Anyway, due to renovations and upgrades taking place at BP, Andrew and Edward have moved their London digs to St. James Palace:
For some interesting (albeit slightly dated) behind-the-scenes background, read Brian Hoey's Not in Front of the Corgis (2013).
I get @taf2002 not caring so much about Angelina meeting with Sophie Wessex, but I posted the news because it's related to an event/ organization that the British royals support. It was not a casual one-off occurrence. I also mentioned the news as an example of the royals not publicizing appearances in advance, which is happening more and more these days. Plus, Angelina Jolie is admired by the Queen for her humanitarian work, and thus Angelina was named an honorary dame of the British empire by QEII in 2014. Since Angelina is not British, she is not officially addressed as 'Dame Angelina.'
Here's an overhead view of Frogmore Cottage: https://postimg.cc/gXCg98C3
Wow, that's some comedown for the Sussexes. NOT! I'll bet anything that the renovations likely will include whatever soundproofing might be deemed necessary. Plus tourists are already restricted to limited visits during the year to Windsor Castle and Frogmore House. I will bet that serious security measures will be taken around Frogmore Cottage, with the tourist footpaths removed and diverted elsewhere. And free rein around the Frogmore House environs will surely be severely restricted going forward.
In retrospect, this engagement pic of Meghan & Harry taken one year ago on the grounds of Frogmore House, near Frogmore Cottage speaks volumes: