If it's anything like mine (Barclays Premier), you just have to start the whole trip in the UK, which you will inevitably do if you're resident here. I've claimed before on trips to Europe where I've flown from London to A, and then back from B to London on a different booking with a different airline (having gone from A to B by train during the trip). At least in Barclays' case it certainly doesn't have to all be on one trip. You'll be fine.