Like all other countries, nothing. But I assume the tax department will do targeted checks on individuals at any time they choose using the CRS and immigration status. Anyone checked, found to be evading tax will then face what ever happens in Thailand.
But in the UK, they go back 20 years. It's assessed tax owed, plus upto a 100% fine, interest at 6% per annum compounded and they will bankrupt you for this money and after in servere cases jail you.