score:5
Correct, lounge access for M&M Frequent Travellers is an M&M specific benefit, not Star Alliance related. So you get access to Lufthansa group lounges when you are flying on a Lufthansa group flight (this includes Lufthansa, Austrian, Swiss, Brussels and Eurowings).
In Heathrow you have access to the Lufthansa Business lounge (you can check the lounge finder at https://www.lufthansa.com/de/de/lounges)