Can I apply for a Thailand eVisa from the UK if I plan to travel to Malaysia first?

Yes, you can apply for an eVisa for Thailand while you are still in the UK, even if you plan to fly to Malaysia first. You just need to provide evidence of your travel itinerary, including tickets from the UK to Malaysia and then onwards to Thailand. Note that UK passport holders receive a 60-day stamp upon arrival in Thailand, so applying for a single-entry tourist visa may not be necessary unless you have specific travel plans that require it.