If a buy an Apple Watch Series 5 from Canada, can I use all cellular or internet features in the UK?

In GPS only mode it can use WiFi and your iPhone's radios.

Basically, in GPS only mode, the Apple Watch can do Everything the activated cellular model can do as long as your iPhone is nearby

Read the following

https://support.apple.com/en-us/HT205547

and pay attention to the foot notes.

Also if you have a Cellular without a contract, then read the document as if you have a GPS only model.

A GPS only Apple Watch works anywhere you iPhone works. But you may have warranty issues.

A North American Cellular Apple Watch will only connect to cellular systems in Canada, U.S., Mexico and Puerto Rico. It will not connect to UK cellular networks. You would have to use it as a GPS only Apple Watch until you returned to North America and signed up for a carrier, and then it would only work while in North America.

It is best to buy your Apple Watch in the country where you plan on using it.

No a cellular version of the watch from Canada will only work in North America,Canada,Mexico and Puerto Rico,if you you buy the GPS only verion of the watch then yes it will.

Stainless Steel only comes with a cellular radio chip.

You do not need to activate the cellular radio, and you can use it as if it was a GPS only Apple Watch.

Yes except the cellular part of the watch.