This isn't necessarily bad news for the UK in the long run. In the short run cancellation of Aukus would mean the UK lost a contract to build submarines for Australia. But tbh I think Aukus was a baited hook that Biden dangled at Boris Johnson, who took it. Unless the UK is really going to defend Pitcairn Island in a war, it has no real strategic interest in having a military focus on the Pacific, and hasn't since the 1950s. The Americans had their own reasons for wanting it.
It's rather worse news for countries in the Pacific region who are alarmed by the increasing encroachments of China - both Obama and Biden wanted to increase American attention to the area, away from Europe. This is a massive signal that the Trump maladministration doesn't give a shit about potential Chinese military dominance of the Pacific. Unlike Europe, there aren't a lot of Western allies who can support a strong military as a deterrent. The Pacific used to be the Royal Navy's waterway, then the US Navy's. Looks like it's the People's Navy's turn.
On the subject of defence, it seems like various countries, especially in Europe, are hoping this Trump thing is just a bad dream. I don't see any indication that they'll make their own defence arrangements just yet. Probably because they're politically unpalatable.
I wonder what would happen if the UK threatened to rip up the Chagos Islands deal and hand the archipelago to the Seychelles with no provision for an American military base there.