Simply put, not much.

Considering that you fly exclusively in J/F and have UC / MLL membership, the following benefits are useless to you:

• Priority check-in
• Priority baggage handling
• Airport lounge access
• Priority boarding
• Gold track

So that leaves these benefits:

• Extra baggage allowance - 1 more piece of luggage than what your ticket allows, but not all flights
• Priority reservations waitlist - if you can't get F for your flight
• Priority airport stand-by - nice to have if you always change your flights, especially at last minute
• Heathrow Express upgrades - useful if you fly to LHR a lot and use the rail

Then there is better IRROPS handling - but as a premium cabin pax, it may not help that much