Perhaps you need to clarify your problem be a bit more specific about routes and needs. Is it that you must not lose any a data link at any time - or could you last without a connection for a few minutes? That might increase the options.

Where do you mean by your phrase "... On the way out ..."? From where to where does this refer?

I am reading your post to mean that you are travelling from Milton Keynes to Stansted for a flight to somewhere else and returning from there to LHR . You can accept the underground on the way back? If you drove from MK to STN why would you need the underground at all on the way out?