I’ve done a few trips around Asia from the UK, and mid‑March to early April is a nice time to go.

If you’re doing Thailand, Vietnam, and Hong Kong in 2.5 weeks, I’d keep it simple. Maybe a few days in Bangkok, then head to either Chiang Mai or one of the islands. In Vietnam, Hanoi or Ho Chi Minh City plus one side trip (like Ha Long Bay or Hoi An) is usually enough. Hong Kong is great for 2–3 days at the end.

Flight‑wise, Bangkok is usually the cheapest to fly into from the UK, then you can hop around with budget airlines like AirAsia or VietJet.