Yeah spam calls are a nuisance in this country. Oddly I've gotten far fewer recently. My technique for answering unknown numbers seems to get me removed from a lot of lists:

Pick up the phone, but stay silent. If it's an auto-dialler, it usually disconnects automatically after 4 seconds, and seemingly gets you removed from the list (as either a dead line or a machine I guess). Sometimes this catches legitimate call centres, but I probably didn't want to talk to them either.

You do not use caller number review websites?

In some cultures, most callers (private individuals) would not start speaking themselves before hearing a sign of presence from the other side: you filter out auto-diallers, but also contacts you did not have in your address book.

In the US the criminals forge the ANI so it looks like a legit number, e.g a local business. We just never answer the phone. Real people can leave voice-mail.

I'm familiar with this custom. Unfortunately, I've adopted the habit of not leaving voicemail (if I'm determined, I talk gibberish until the callee picks up).

And I haven't listened to recorded VMs for years. Way back when, I actually bought a machine for answering phone-calls. I have no idea what I was thinking :-)

Voicemail transcription is the only reason the "ignore it and if it matters they'll leave a voicemail" strategy works for me. I never, ever checked voicemails back when I had to actually listen to them. Now I can skim ten of them in a few seconds to see if any were legit. Most scammers/spammers don't leave a message anyway, so the volume's pretty low.

They seem to use such varied numbers that phone number databases are no longer useful.

I think the UK is a hot spot because its population speaks English, which means the same scammers that target the US can easily broaden their target market.