Electrification between Didcot and Oxford has been delayed until 2020, but one of the plans is to keep either the 5 First Group owned HSTs or an allocation of 16x for Oxford Fast Terminators, with 800s covering the diagrams that continue on the Cotswolds
A shuttle provided between Oxford and Didcot using 16x to connect into 387s on the stopping services is also written into the plans. Platforms 4 & 5 set to be used at Didcot for this to happen to enable cross platform transfer.
GWR wanted to avoid this, but the DfT deems mainline electrification more important
I can't think anyone will want 90mph Turbos anywhere near the fast lines if they can help it, given the need there will be to give the 800s as much 125mph running under 25kv as possible to minimise the impact on timings of the extra diesel-mode working further west. HSTs will offer the speed plus plenty of seats and allow more Turbos to go to Bristol, so surely are the better bet, albeit with higher running costs.
