Many UK banks require minimal monthly "pay in" (often 500-1500 GBP) to get interest rate payed on their current accounts. Sometimes there is a restriction that payment must come from another bank (payments from other accounts in the same bank do no count). At the same time, as far as I know, there are no restriction for having several current accounts in different banks.
So it becomes quite easy to formally arrange the required "pay in" with having simple standing orders transferring money between one's accounts in different banks (like A->B; B->A). Of course this trick requires having at least 500-1500 GBP on those accounts.
The question is: why do banks still require that "pay in" if it can be easily implemented without actually paying additional money into the system? What is their reason? Where is the catch here?