louie-m is correct, it's once per year (I can't remember whether it's calendar, membership or rolling year, I was asked if I'd recently made a transfer, but that was it), and you can pick what you transfer.

I kept about 50% of mine in the UK scheme, I keep a balance there so I can get Avios easily (as they're not easy to come by in Australia). The upcoming devaluation of our MR programme prompted me to pull the trigger now.