You have a very optimistic consideration of the capabilities of HMRC and of their priorities. There is a lot that just flies under the radar, because it’s just not worth anybody’s time to pursue it and it’s hard to impossible to actually identify. Information exchange with other countries is very, very limited, and likely decreased since 2018 as well.

Let’s say I move to Spain and start a business there cash-in-hand. I declare to HMRC that I earn nothing and I reside in UK. I proceed to collect UK subsidies, discounts on council tax, and various other benefits. I might rent out my house as well, either legally or illegally. Would I pop up on any radar? Unlikely. HMRC has no visibility of Spanish transactions and I don’t appear in any UK transaction, as far as they know I’m just a bum. If pressed by Spanish authorities (big if) I can just say I pay taxes in UK - which is “true”. If business in Spain does well enough I might just stop taking UK subsidies, to avoid scrutiny even more. If I never had to file self-assessment before I shipped out, I’m basically off the radar.

That wasn't my experience as a UK expat in the Netherlands. I was there for 11 years and I never heard a thing from HMRC.

If you were paying tax in the Netherlands they would have automatically notified HMRC.

That explains it.

Presumably you were paying tax in the Netherlands though?

This has never happened to me either, notified HMRC and that was the last I heard.