ITN have the studios they have because of cost cutting. ITV News London I am sure would like to have their own dedicated studio, which they had from 2004-2012, before being lumped in with the national news bulletins.
Even if there was a free studio available the London news would still share a studio with the national news. It's all about saving money, running two studios unnecessarily is far more expensive than running one.
ITN are not opening other studios because they don't need any. And GMB and Lorraine couldn't "easily" move in, they need a lot more than just studio space.