Higher prices for food and drink usually bought for half elsewhere.
A chicken tikka pitta bread is a fiver on LNER. With that fiver I can buy a much larger portion of food from a full large meal at KFC, McDonald's, or Burger King, including a drink. When time is a luxury, it's a no brainer. When time is constrained by being on a train, then the buffet becomes the alternative.
In my younger days when the southbound Invernesss/Aberdeen to KX arrived at Waverley on platform 2 and had 10 minutes, My parents would task me with bolting to Burger King and coming back with 20 quids worth of chips, burgers, and chicken for 4 of us to dig into. Add an extra 3 quid on for sweets 2L drinks bought at Asda before boarding the train an hour or two previous, and we were stuffed for the rest of the day - at £5.75 each. Compare that to the above and anyone can see the dillema. £5.75 for my ration of a heap of chips, burgers, chicken, snacks, and a soft drink. Or a Chicken Curry Pitta and a packet of Polos I can buy for that price now? No brainer.
For buffets to maximise what they take in, they need to be inovitave with their food options, deals, and advertising. Hence why the likes of LNER and Virgin WC are selling food like Mac n Cheese and Chicken Curry. Not commonly sold hot and prevelant fast food outlets, but can mainly be found ina reheatable state at coffee shops for similar prices - which I imagine is the only real place any buffets can compete with price wise. They are never going to compete for profits anyway, but the least they can do is lower the subsidy to make it justifiable to the TOC.