Potential international speakers should also be warned this is also true of the US - except for certain types of academic institution (INA 212(q)). The UK also allows it via https://www.gov.uk/permitted-paid-engagement-visa but again, only for arts or academia. A commercial conference doesn't count, annoyingly (or even a community conference that merely happened to offer honoraria). However, if you are being paid by your employer to attend a conference to speak, it is fine (big disclaimer: IANAL).
Related from a few years ago, Uncle Bob was turned away from the UK: https://news.ycombinator.com/item?id=3282583 - but I'm pretty sure he has been back since.