I don’t work for the railway and the professionals will know what they’re doing but this is what I’d propose:
Worcester Foregate Street/Shrub Hill to Reading formed of a 166. Possibly the 06:53 from Worcester to start back from Hereford if possible, with one of the last trains of the day running there as there’s not a great deal of services around these times and so staff don’t lose competency if this is going to be long term. Passengers can board WMR and change for the rest of the day.
London Paddington to Swindon formed of Class 387s, with the aim of them being extended to Cardiff in the longer term, subject to staff training and clearance.
Swindon to Bristol to be operated with whatever’s available; whether that’s 80x, Turbos, Class 158s, HST GTis. There used to be a Turbo service that ran Gloucester to Swindon via Bristol until recently so at least some depots should sign both the route and the stock.
Swindon to Gloucester to be formed of a 16x or a 158, whichever is available.
London Paddington to Swansea to operate as a class 80x between Swindon and Newport or Cardiff only due to a shortage of units. 158s/16x could also cover if needs must, they’ve done it before on diversion...
London Paddington to Plymouth/Cornwall to run London - Newbury as a 387. Turbos to operate Taunton to at least as far as Newbury, extended to Reading if possible, calling at Castle Cary, Westbury, Pewsey, Bedwyn, Hungerford, Kintbury, Newbury (and Reading if possible)
Turbos used as normal on Thames Valley branches.
Some 150 services split where possible in Devon, with 150s covering for 158s to Barnstaple, to free those up to work other services or to release Turbos to work other services.
Not too sure what can be done about the local/regional GWR trains around Bristol. Weston-super-Mare to Bristol Parkway could have stops picked up by Cardiff - Taunton trains to free up a couple of units. Some of the extra peak time trains via Bath and Westbury could probably also be dropped, but many form other things afterwards so that may cause more problems than it solves.
Cornwall branches to stay as now but the mainline to receive current mix of 158s and HST GTis.
Then there is the potential of XC helping out as per now, borrowing stock from other operators as previously mentioned (even if they do have to run at a lower speed), plus the 80x gradually returning to traffic, Chiltern running more frequent/longer services from Oxford etc.
Of the IC routes, I’d say the below is the priority list of which services need to be restored first...
1. London to Cardiff
2. London to Bristol Temple Meads
3. London to Plymouth/Cornwall
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4. London to Worcester/Hereford
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5. London to Gloucester/Cheltenham