I'm still confused. So, some questions:

First, which UK property is this? How about some details on the rate? Depending on the rate and property it might or might not be obvious that there was a mistake.

Are you saying that they are refusing both dinner and breakfast, but you are only concerned with compensation for dinner?

You mention an email you received yesterday; was there something in the email notifying you that you would not receive the dinner and breakfast which was included in the rate you reserved? If so, what prompted the email to you?

Does the original reservation (which, I assume, you printed out) specify dinner and breakfast included in the room rate?

When you booked this rate, was it, or should it have been, obvious to you that the rate was a mistake, or did the rate seem reasonable, even if a bargain?

Depending on some of the answers, if you have it in writing, I would deal with the hotel G.M. and insist on dinner and breakfast. I'd point out that there was no way you could have known the rate was a mistake (assuming that's true), and that you only made the reservation because it included dinner and breakfast. I wouldn't bargain with them, but might listen to compensation offers from them if they were rich enough. As to amount of compensation, you might start with the price of dinner and breakfast in the hotel, and convert to points at, maybe 1-2 cents per point. (if dinner and breakfast for 2 is valued at, say, 200 GBP--this would roughly convert to $300 USD; about 30,000 Marriott points.