It's also worth taking flasks - one or more for hot drinks, one for keeping a pint of milk cool in. Sat in cold water in the sink also helps if not. Buying a pint a day should be possible and still way cheaper than buying mugs out - get everybody to bring thermal mugs and all take a mug full out for the day, plus flasks.
Bonus points if you can get hotel to give you a scoop of ice from their machine breakfast and night for the milk. If you explain your prefer fresh milk in t&c in your rooms they'll often offer you a small jug to take upstairs anyway.
But if cost and convenience when out and about is the main concern, I'd find a local shop or supermarket and start the day popping by there for a loaf of bread, pack of cold meat, cheese, butter and multi pack of crisps. Maybe a 2L bottle of coke . Put cold things in cool bag in car until wanted, then use picnic plates etc (take sharp knife for cheese!) And have, well, a picnic!
You can mix it up each day - say ham, gouda and tomatoes one day, corned beef, cheddar and pickled onions the next.
Loaf of bread £2
Packet of 5 slices ham £2.50
6 pack crisps £1.50
Butter block £2
Chunk of cheese £3
Bottle of pop: £2
Pack of tomatoes, apples or pickles £2.50
I make that £15.50 or half the cost of what your said buying it all out would cost 
Pro-tip yes I have self catered in hotels and B&Bs a lot why do you ask take a small container of washing up liquid. I refill a mini shampoo bottle.