One transaction a year is all that is necessary to keep your bank account active. So just use your banking app to do a small deposit or transfer. But beware, if you neglect to do so and the account becomes inactive you can only activate it again by physically visiting a branch office with your passport.
Thai Banks don't use Iban numbers. If you make a telegraphic transfer from your UK bank to a Thai bank account all your bank wants to know is the name of the Thai Bank, the branch address, the name of the account holder, the Thai bank account number which is not iban and the swift code.
a Swift code is not made up from a recipients IBAN number. It is quite separate. For instance the Swift code for Kasikorn Bank is KASITHBKXXX. The Swift code for Krungsri Bank is AYUDTHBK. In fact Banks in Thailand do not use an IBAN (International Bank Account Number). However, they do use the SWIFT coding system for banking transactions.
no Swift is not an app. SWIFT is an acronym for the Society for Worldwide Interbank Financial Telecommunications. Banks including your UK bank use the Swift system to transfer money internationally. My bank which is Nationwide charges £35 no matter what the amount. So I don't know where you got the figure of £600 from for the transfer fee. And when you send it make sure you send it in pounds to be converted here, not Thai baht from your UK bank otherwise you will get a really bad rate.