Could pre-ordered (higher quality) microwave meals be provided (weren't XC trialling this pre-Covid?). Most express passenger stock has a microwave, so multiple operators could be supplied at certain node stations- eg Reading, Birmingham, Glasgow, York, Edinburgh etc. This would allow a fairly widespread offering across the network, with little wastage as meals would be pre-ordered and pre-paid. It would have to be delivered by existing staff though that are already onboard to make it a revenue positive and productive solution however, which is probably the only way you could realistically 'sell the idea' to revenue-conscious decision makers. That way most long-distance trains would have a decent catering offering open to all.