Are you aware that electrification was considered and rejected for:
Selby to Hull
Oxenholme to Windermere
Kettering to Sheffield
Cardiff Central to Swansea
And that work has been suspended on:
Oxford to Didcot Parkway
Bath Spa to Bristol Parkway via Bristol Temple Meads
Now keep in mind the schemes currently still "on track":
Maidenhead to Cardiff Central
Reading to Newbury
Manchester stations to Blackpool North via Bolton
Edinburgh to Glasgow Queen Street
Croy and Falkirk to Alloa and Dunblane
Cumbernauld to Grangemouth
Bolton to Wigan
Barnt Green to Bromsgrove
Walsall to Rugeley Trent Valley
Proposals are also due for the TransPennine Route Upgrade, these may or may not include electrification.
Now, after all of those where do you think Worting Jn to Exeter St Davids might fit in? How many trains and passengers traverse the route? How many new trains and of what type would be required, and what benefits might this have in terms of releasing the other stock? What would the effect on journey times and running costs be and critically, is the track layout and other infrastructure in a condition suitable for the long term (before electrification which makes long term changes more difficult) is considered? Then, what type of electrification would you be proposing to use and why? Would that create any problems and how might they be solved?
Only once you have considered all of those things can you answer questions about the interior layout of the rolling stock would be like. I am not saying this is a bad idea, I am merely saying that you might be able to think of some of the answers to questions in this thread yourself by looking at it in this way.
Edit: PS it's also worthwhile looking at historic schemes like:
Cambridge to Kings Lynn (1992)
Redditch to Lichfield Trent Valley (1993)
Leeds and Bradford Forster Square to Ilkley and Skipton (1994)
London Paddington to Heathrow Airport (1994)
(those were pre-privatisation though, so of limited relevance)
Kidsgrove to Crewe
Glasgow Central to Paisley Canal
Springburn to Cumbernauld
Glasgow Central to Whifflet
Manchester stations to Edge Hill via Newton-le-Willows
Were they successful? What could make them better? Who provided funding and why were they authorised?