At a Local forum this afternoon, GWR▸ were insistent that the Castles will be withdrawn on the current expected date, so it looks as though the 175s will start...
Or, as discussed, other trains will be shortformed. There are still some trains running around as MULTIPLE units (
IETs▸ , Turbos and sprinters) - and it would be possible though unpleasant to inadequate - to provide shorter trains on some services.
If it was November or January in South West then might be acceptable. But in May as busy tourist season starts is not a logical time to cut formations.
Of course there is Glastonbury (25-29 June) when other services have been cut in past to provide trains. Not that GWR keeps extra stock for its 140+ days annually of busy trains (Cheltenham festival, Henley Regatta, Cornwall summer school holidays, Bath markets, Cardiff rugby specials etc