How old are your grandchildren? I was wondering if they had email and you could keep in touch that way. My grandchildren live locally to me but I sent them lots of emails anyway and now they have iPods we FaceTime each other. They like links to funny things on youtube or I take photos of things that might interest them and send those. If no email then how about sending funny postcards. You might not get a rely but the contact will be there and the children more disposed to you when you do see them.
To think that London, or anywhere else for that matter, does not belong to any one demographic
Adult early 30's - very difficult.