One of the station name boards at Bere Alston (Tavistock end of platform) was still showing Wessex Trains branding in April when I last used the station.
I took this photo of it in September 2021
I think like several others I've seen it was covered over at one point with a cheap First Great Western sticker but has been peeled off.
I remember at Oakham station a contractor when East Midlands Trains had the signage redone about 10 years took their brief to take note of and replicate the existing signs too literally. An EMT corporate style sign appeared advising that the British Railways Board gave notice that some out of date bylaws applied to car parking. It might even still be there, it was attached to the back wall of the Leicester bound platform.
At least 1 EMR 170 has a Central Trains sticker in the cab.
158889 has South West Trains branded signage all over the cabs.
Alfreton has a Regional Railways free standing poster board and loads of EMR stations have the Regional Railways leaflet racks, including Loughborough and Long Eaton, with the branding still intact.
Codsall has a BR Western Region original enamel brown sign advising you to only cross the line by the footbridge (it was there before the pub conversion and faux old fashioned signage appeared).
Rauceby signalbox has a Regional Railways sign on the elevation that faces away from the crossing.
Harling Road has a Regional Railways running in board.
An EMR 158 has a Regional Railways Alphaline poster from about 1994 from the fleet engineer at Crown Point regarding resetting the air conditioning.
One of the ex Wales and Borders 158s at EMR has "Don't forget Oakengates!" drawn in biro on the driver's sun blind.
There's plenty about if you look, mostly in obscure places that have been missed but also some surprisingly obvious ones!