What travel tips and COVID-related advice should I know for my trip to Samui and Phuket from the UK?

To keep up to date with Thai immigration news.

That's the most important.

With the emergence of new variants things here can change with little to no notice.

Thailand if things get very bad have no problem in introducing 7, 10, 14 day quarantine again with absolutely no notice.

Also Keep upto date with your home countries re-entry rules.

For example UK just re-introduced testing for all international arrivals which could change to 10 days expensive quarantine for all arrivals if the Omicron virus gets a foot hold in other countries and turns out to be the nasty variant that everyone is speculateing it to be.

Just be prepared for cancellations, delays and added expenditures as worse case scenerios....and then u will be fine.