Looking at the PP app, the IASS Executive Lounge seems to be land side, where as the IASS Superior Lounge is airside. So just use the landside lounge and save some of the trouble.

It is easy to manage the transfer restrictions, just ask to see the boarding pass. Heathrow stopped allowing inter terminal transfers without a boarding pass for the relevant terminal. I guess too many people getting caught by the transfer time needed and missing flights.