It depends entirely on what you want to get out of the FFP, and what your flying pattern will be. There are three main benefits available:

1) award tickets (eg free tickets)
2) status with the airline / alliance (ie lounge access, fewer queues)
3) upgrades

bmi is a good FFP for (1) above as it allows miles + cash redemptions, which are generally considered good value. It's also good for (2) above being relatively easy to achieve and maintain gold status with star alliance (especially if you're flying in premium cabins - will your (presumably) AUS - UK trip be in economy class?). It's fairly hopeless for (3), though, so if you're wanting to upgrade between UK and AUS, you may be better with UA (although you'd need to fly the wrong way round) or LH (with an extra connection).

Search this site (or, indeed, google) for BlondeBomber's Star Alliance Comparison Chart, which does pretty much what it says it does.