Wriggling in and out of Westbury slows the train down by rather more than any of us like. And there's 3 minutes of recovery time and 4 minutes of station stops in the schedule. We were lucky to get a clear run to Bristol where I left the train, it would seem there was a clear run beyond, and probably beyond judging by the timings. And I suspect the 125 was in top form and not being driven to economise on fuel on this occasion.
Silly comment on Westbury ... the avoiding line's only 240 metres from the station - that's the length of a 9 car
IEP▸ . There are already examples of a considerable passenger walk to get around stations (has anyone here every travelled on a 12 coach train from King's Cross to one of the new platforms in Cambridge?, then exited the station there?). Couple of extra platforms on the avoiders, anyone? Solving capacity and run time issues ... Just doing an airline comparison, it would appear that the longest walk (platform -2 to +3) at a Westbury would be less that the shortest walk you'll have from road/railside to your plane at Gatwick South - excellent diagrams at
https://www.gatwickairport.com/globalassets/documents/passengers/prm/58634_prm_walkingdistances_v4.pdf