It was previously mentioned on this forum that the catering layout was the decision of the TOC.
That depended on what the DfT was willing to accept on 'their' trains and with the first batch, going to a franchise currently run on a direct award basis, they weren't budging from what they had decided they wanted, whatever GWR said. The ECML is run by the winner of a competitive franchise contest, where the bidders were asked to promise all manner of goodies and, given their trains are later down the production line, they appear to have been given room for manoeuvre on what they do with the interiors.
We have yet to hear what sort of layout the AT300 sets GWR is acquiring via Eversholt leasing are going to have but with today's big plug for Pullman restaurants in the brochure GWR has put out, the full kitchen seems a certainty in that fleet too. Though hopefully a rather more attractive colour palette will be used for the interiors and seating than DfT's sea of grey.
Not sure where you get those figures from. A 221 seats 230 standard class passengers (245 if Coach D didn't entirely consists of table seats). A 5 car 800 seats 270 standard class passengers (with a high density seating arrangement), according to the draft seating plan. So a difference of 25. Considering they are longer trains - and don't have an 'on-board shop' - I'd bloody well expect them to accommodate more passengers!
I believe the DfT seating layout has been adjusted recently to have more standard seats than on the plans that are online, so more like 280 seats, which is 50 more than a typical Virgin 221, and with eight tables of four seats in a standard open coach versus all of two in a 221 - and they aren't crammed into a cramped tilt bodyshell and the toilets might just not stink.
Almost two decades later, I was hoping we had learned not to create legions of wasted space, a la Coach K on 390s. If a 5 car 800 has 4 full standard class carriages - like a 221 - it will seat 320 standard class passengers.
There are some posters on this forum who would find the suggestion that any sort of catering facility unable to deliver a full Pullman meal on a GWR service would be inadequate, so how can the space be wasted...?
And how else can Virgin West Coast deliver first class meal service on its trains without the 'wasted space' in coach K on a Pendolino?
If GWR does get DfT consent to order a few more AT300s tailored to the needs of Oxford/Cotswold/Newbury services, then you could see five-car sets with a simple galley in a first class-only driving car and four full standard coaches. Time will tell.