I am not solicitor so this is just from my own experience/understanding. The distinction between a home owner rate and a buy to let rate is not based on legislation it is just marketing talk from the bank.
However if you told the bank that you were buying the house to live in and they offered you a rate based on this, they could claim that you have broken the conditions of the contract and they want to put you on the buy to let rate. It all depends on what is in your contract.
I think they would have no chance of persuading a judge that they are entitled to put you on a higher rate retrospectively.
I also think that the bank are unlikely to be looking for problems, why would they want to put you on a higher rate which you cannot pay. Bad for Mr or Mrs bank manager, another customer going into arrears.
Also you dont need to signal to them that you are in the UK. If you tell them your salary you dont have to provide payslips, even if they would like that.
I believe that, at present at least, if you simply pay IO the banks will not take any action against you, even if they refuse to "agree" this and they huff and puff for a while.
My advice, pay IO, pay some extra as a capital payment occasionally, and ignore any correspondence from the bank unless it is legal action, (not just the threat of legal action).