Fist Class still exists on SWR, GWR, LNR, Thameslink, Greater Anglia. What is a local/commuter service = are you including Cambridge, Peterborough, Milton Keynes, Northampton, Didcot etc in that list?
If we use Chiltern as a benchmark:
LNR: Abolish entirely other than Crewe semifast, on that refurbish some dedicated units with proper 2+1 1st, or if that's too operationally awkward either get rid of it there too or declassify other than on Crewe services.
Thameslink: Declassify and take doors off, just keep it as a "more comfortable seats nominally for people going further" area. Self-policing as e.g. people starting at Bedford/Brighton can sit there and then it won't be available to others. Half of it already is that anyway!
Greater Anglia: only the IC FLIRTs will have it anyway once the fleet replacement is complete
GWR: 80x services only, which is already the case, the EMUs are Standard only
Thameslink: Only on services to Weymouth and Salisbury/Exeter. On 450s, convert to Standard or declassify with doors removed. On 444s declassify when they work anything other than a Weymouth. On 458s do not install to start with. Self-policing as e.g. people starting at Bedford/Brighton can sit there and then it won't be available to others.
Southern/Southeastern: declassify and remove any doors most if not all of it is the same as Standard anyway
TfW: Manchester-Cardiff and "WAG Express" only, otherwise declassify if present
ScotRail: keep on Inter7City services only, remove on all other services including Glasgow-Edinburgh EMUs
Chiltern: declassify on LHCS, already not there on other services. LHCS hasn't got long to go anyway.
TPE: Consider declassifying where TPE are operating stopping services basically on behalf of Northern. Keep elsewhere.
Keep all 1st on:
LNER
Avanti
Due to the reduced number of First Class tickets around as a result of this, it should be easily possible to purchase an upgrade to an existing Standard ticket at any sales channel, priced commercially as the relevant TOC wishes, or 1st could even be considered an "Advances and upgrades only" product with the base fare set all Standard only.