Yes from the UK and all other EU countries its only 30 days.
Which lucky countries recieve 90 days on arrival?
The ones I've got here now are from Brazil, but it's also true for other countries, such as Korea. It's a question of bi-lateral agreements between the countries. Further complicating the issue is that citizens of some countries do need a visa, but can get it as a 'visa on arrival' so that they make application at the airport at which they enter Thailand (visa waiver is different--just an entry stamp in the passport)
For some countries a visa must be issued at a consulate outside of Thailand. For a subset of these, a health clearance is required before the visa is issued.
For other countries, visa waivers are only given to diplomatic/official passport holders (usually 90 days). Everybody else needs a consulate-issued visa.
For a small number, a visa on arrival for only 15 days is granted, no matter whether the visitor arrives by air or not.
For a group of countries (including the EU) the visa waiver is 30 days. For some others it is 90 days.