For top-notch security developers, I call bullshit. Signal would be worthless if it started offshoring development to nickel and dime.
I don't think there's industry numbers for that set of people in the UK, as it's not a big enough set. However I'd be surprised if they were 150K plus though, that's a very rare salary in the UK.
Also there are cheaper countries than the UK who have great devs.
The people behind Signal pioneered end-to-end encryption, and as is pointed out in the blog post, there's still a lot of novel cryptography development involved in building a privacy-first messenger. You can't do that without top-notch talent.