Stratford from SW London isn't that bad, nice fast tube from Waterloo, and soon on Crossrail I think.
If itv does set up in Stratford its very unlikely they'd have a general purpose studio like they did in the south bank
As well as Sky, Elstree and Pinewood are pigs to get to
I don't know about pinewood and Sky as I have never been to a show in either of these studios, but Elstree is not that hard to get to from Central London, We stayed there a few years back and used it as a base to visit London to, its has its own rail station that is the last on line you can use your travel ticket on, as I don't like driving in London we parked at the station, and then travelled into London. but we could have also used a good local bus service, but walking from the station to and from both the BBC Elstree site and Elstree Studios is also possible as its not a long distance to them.