Not sure of the policy but I am sure that practicalities and timescales come into it. Not to mention that there will be plenty of consultants, advisors and other corporate travellers who will not fall under the civil service policy if part of the treasury were to relocate to Teesside.
If you were travelling by train to Westminster from an office near Teesside airport I guess your timings would be something like: leave the office 30 minutes before the train leaves Darlington, hopefully 2 hours 30 mins on the train then 30 minutes tube/taxi to your destination (so 3 hours 30 mins total).
If you take the plane then you would leave the office an hour before departure, 1 hour flight then 1 hour Heathrow Express/Tube or Taxi (so 3 hours total). In that example the train wins despite (taking 30 minutes longer) on environmental grounds, ability to work whilst travelling and flexibility due to sheer number of trains between Darlington, Eaglescliffe (and soon Middlesbrough).
But if you are going to Slough, the dynamic totally changes. The train will be something like 4 hours door to door and the flight 2 hours 30.
Of course price can also come into it. An early train to London is generally not so cheap if you are booking under a week in advance.