They did do some pickup shots of Attack of the Clones at Elstree, but as far as I'm aware that was mostly because the stages were being used up in Australia (where the majority of StarWars II and III are being filmed).
It could have also been because of Lucas' own superstitious beliefs that maybe some of the luck of the original films might rub off.... who knows.
I do know they have no plans to do any pickup shots there for Episode III, they've booked up the studio in Australia for next year for that.
It's unlikely they'll shoot any further Lucasfilm stuff in the UK anymore. Financially it's unviable, most of the studios in the UK are very busy and studio space is at a premium.
You also have to remember, many of the people, including Ford, Spielberg and Lucas all have families now and more than anything are doing this next film for fun. It's a lot less fun and a lot more inconvenient to travel half way across the globe to shoot something they could do in their own backyard.