Firstly, it's very difficult to make it profitable, given the limited number of passengers you can serve on a relatively short journey, the staff costs of running a kitchen plus waiting staff for ~10 tables and the limited price you can charge before the reputation of the train company starts to take a hit for "excessively expensive food".
Secondly, British trains on popular routes are now overfull. Capacity is bounded by trains/hour (fixed), maximum train length (fixed, by signal block and platform lengths), and capacity per coach. So we're optimising for the latter: thin seats to maximise density and a loss of luggage vans, kitchens, extra toilets, and other facilities that take up space but don't seat passengers.
The modern standard is a catering trolley that serves coffee and snacks at your seat. Because it lives in the aisle and moves, it doesn't take up revenue space. Because it mostly sells coffee and alcohol, it turns over a lot more per hour with only one staff member.