Perhaps the extra 802s are being ordered to allow the Turbos to be cascaded from the Oxford-Paddington stoppers before the delayed Oxford-Didcot electrification is complete.
Why? The stoppers tend to largely empty out and refill at Didcot from either direction - so a break of journey there to change between a 387 and a Turbo shuttle for the Didcot-Oxford leg for a couple of years is not going to cause too much grief. Most stoppers already have extended pit stops at Didcot anyway, so the impact on journey times of the change would be near to non-existent.
The service could run as a stopper between Oxford and Reading, and then fast to Paddington. This service could be operated by 800s displaced by the extra 802s.
Some of the 387 services are likely to do just that anyway, same as happens with services worked by 16xs, 180s and peak HSTs now - just that the electrics will run to/from Didcot until 2019.
There would probably be demand for the 802s to be transfered to other routes once Oxford-Didcot electrification is complete and the service can transfer to EMU operation.
You seem to be forgetting that the 387s are now going to be covering Thames Valley services that were previously planned to be the domain of 365s, which leaves fewer 387s - even with the boost to their numbers - to do things like work Oxford fasts - which is where the extra AT300s come in.
And these extra AT300s will scarcely have arrived in GWR territory before the wiring to Oxford is completed in the spring of 2019. What would the point be of ordering them if they were going to become redundant there a matter of weeks after delivery?
There has been strong growth in passenger numbers, ahead of the national average, on the London-Oxford-Cotswold Line axis for quite some time, so allowing extra capacity to allow for that trend to continue makes sense and having extra 80x trains will also allow GWR to provide a consistent product on Oxford-London fasts, with an 800 or 802 on services that turn round at Oxford and those running to/from the Cotswold Line. Building on the move by GWR only this May to have more 180 and HST turns on the fasts that turn round at Oxford, instead of 166s.
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