I expect the rear end didn't clear the track circuit locking the barriers down.
I've heard that GWR^▸ (Great Western Railway) tested the HST^▸ (High Speed Train (Inter City class 43 125 units)) to be used as final resort when no other stock is available.
I saw it at St James' Park where it stopped for 45 seconds. The front loco was on the platform stopped at the stop board. 4 staff got out from cab, front door or rear of car 3 just on the end of the platform, looked, waved and got back on again. Departure was very sharp I guess to force the acceleration to test it against the more nimble 150s & 166s.

There is a shortage. Several days per week Exmouth-Paignton sees two 3 car 166s and 4 car 150s. But too many days a missing turbo or 150 results in two 2 car 150s or 2 or 3 car 158s and rarely a 2 or 3 car 165. Even full length trains can have standing in the good weather.