With the 5hrs max you have I really don't think its worth going into London takes at least 45 mins on the train each way and that's if you don't run into any delays on the way there or back.
I think if the weather is ok then get some pictures the problem is at the moment the sun at lunchtime and in the afternoon is sitting at one end of the runway which doesn't make for great photo opportunity's (at least not with my lack of skill !! ) the sun is facing pretty much straight down the length of rwy 05 and seem to be using 23 most of the time at the moment.
You could walk down from the terminal towards the SR
technics building and round towards the hilton and get some pics from there. I guess that would be about a 30 - 40 minute walk.
I have never been the other way from the terminal past the radisson so someone else will need to let you know about that.
Hope that helps you a bit more