I just got back from a weekend trip to London, as commentors above mentioned it's legal there. Which is good and bad - good because the pics/videos/reviews that you seen on the websites are generally legit, and there are a shit ton of them. The bad news is that there are plenty of hustlers out there, especially at the lower end of the range.
Here's my experience with some various agencies at different price points:
In general: very easy to book. Just whatsapp their number and request from there, I was able to book several days in advance. I even got an outcall for my hotel at heathrow who was pretty much waiting for me when I landed.
Angels of London - this was the best experience I had. Price was comparable to NYC.
Dior Escorts - also had a good experience here. Similar price as above.
Platinum Escorts - I got bait and switched by this agency on my last night. It was the cheapest by far (advertised as a Heathrow outcall specialist) so I don't necessarily regret it but the girl they sent was nothing like the pictures at all. Totally different girl, had body odor and smelled like cigarettes. This was an anger fuck.
Bubbles Escorts - another Heathrow outcall agency, the girl they sent this time also didn't look like the photos but we vibed pretty well. So pretty hit or miss.