Technically. I spoke to the Guard on all SWR services I travelled on from home to wherever it was I caught the GWR services. Always prepared to pay, but most wouldn’t sell me a ticket. The one occasion I had to get a XC service from Basingstoke to Reading he sat me in First Class and told me to help myself to a brew
The first time I travelled to Swindon after my local induction, the administrator at the depot gave me a travel warrant so I didn’t have to pay on SWR. I didn’t need to use it though. It’s again one of those contentious points, but it’s general railway etiquette to speak to on board staff as a courtesy and be prepared to pay for a ticket, but even showing a temporary pass will usually result in being waved passed and told to make yourself comfortable.