there is no F lounge in LTN
Seriously, only flights to IOM from Luton or Gatwick (or MAN, obviously), so no zone R but you do get into the F lounge at LGW.
[and the anomoly is apparently because 4 round trips LGW-IOM is slightly too long to do in one day for the crew, but 3 LGW-IOMs and 1 x LTN-IOM cuts the time down just enough for them to stay in hours]