I think i'll be staying in Chiang Mai, Phuket and Bangkok. I'm from the UK.
Britons can look to the NHS's Fit for Travel website, which provides detailed health recommendations for most countries.
NHS recommended immunisations for Thailand include the following:
Confirm primary courses and boosters are up to date as recommended for life in Britain - including for example, vaccines required for occupational risk of exposure, lifestyle risks and underlying medical conditions.
Courses or boosters usually advised: Tetanus.
Other vaccines to consider: Cholera; Diphtheria; Hepatitis B; Japanese Encephalitis; Rabies; Typhoid.
Also see the guidance on the risks of malaria, dengue, and zika, among others.
CDC has pages on this for many countries. For Thailand, they recommend Hepatitis-A and Typhoid, in addition to the "routine ones" everybody should have.
And then, depending on where you go and what you do, they offer half a dozen others.
According to Spain's Health Ministry (link in English but recommendations in Spanish :| Spain, you know...):
- Mandatory vaccines: yellow fever vaccine for those coming from places with yellow fever transmission risk. You are from the UK, so it doesn't apply to you.
- Recommended vaccines: just the ones already included in the official vaccination program. Your family doctor might recommend additional vaccines depending on your health record.
- Paludism: while most of Thailand is safe, there's risk of paludism transmission in the borders with Cambodia and Myanmar, so you are warned to take precautions if you are travelling to those areas. Most cities like Bangkok, Chiang Mai, islands like Phuket, etc. are safe.
Now to the personal experience: I've been to Thailand without taking any additional vaccines beyond the ones I already took 20+ years ago. It was a (wonderful!) road trip: several places, rural villages, some street food, some wilderness, etc. We had zero problems regarding diseases.
However, we did have a medical problem, because the sun in Thailand is freaking strong -- even for a Spaniard used to 40 Celsius in summer. My wife got a heat stroke! So make sure you wear sunscreen at all times, a hat, and even a long-sleeved t-shirt wouldn't hurt either.