If I recall, wasn't one of the reasons for the delay to the Maidstone service to do with a new depot at Ashford. I guess that could actually be an incentive to sort it if the intention was to relieve the pain on three bridges.
For interest Hornsey is a fairly major depot, what can three bridges do that Hornsey can't? (Thinking that the units allocated there are shorter so it could be as simple as they can't work on an 8 car sets. The 387s can go as 4 and the 717s are only 6 cars. But that is the reason for the question, the 717s are not miles apart from the 700 are they mechanically?