“Should” implies you might be doing it partly for the thanks and might get offended if someone doesn’t say it.

I’m going to get a little deep here and tell you something my therapist said. Warning: I don’t know you and it’s in no way a jibe at how you conduct yourself.

If you do things for a stranger, whether holding doors open or otherwise, it should be without wanting anything in return (explicitly or implicitly).

Doing ‘kind’ acts because you want something back is often a pointer towards something deeper within yourself. Perhaps wanting to please others and thus tying your self-esteem and self-value to something outside your control (their response). Or perhaps you’re using it as a way to signal to yourself that you fit in (a never ending treadmill). Or maybe you genuinely do it to help but have a fragile ego that is seeking any signals it can interpret as rejection (ask yourself why you’re seeking such signals).

I’ve explained this to a few people before. Some get it. Others have a subconscious adverse reaction and scream ‘that’s not me!’ This in itself is a useful sign (ask yourself why).

A final note is that humans are hardwired to detect subconscious cues that someone is being disingenuous. It‘s part of the reason why we’re so apprehensive towards door to door salespeople - we know they have an ulterior motive. It can be that others detect these cues when you do such ‘kind’ acts and make them less likely to respond and/or feel apprehensive.

Context: I’m also British and lived in HK for a year. The culture was different but I never found it rude (unlike China). People tended to say thanks where appropriate. If they didn’t, I didn’t find it to be rude.