For the details, you can check the East Coast timetable, which shows which meal service you'll get.

Scroll down to your departure point (Doncaster in this case) and note the colour of the highlighting, bearing in mind that you're only entitled on journeys over 70minutes. (Fine in this case)

It looks like you will get 'Chef-Cooked' Breakfast up to the 1046 departure, then one sole train (1058) gets Breakfast (no, I'm not sure either). After that, it's the 'all day' selection (sandwiches or a hot dish), with the exception of a couple of departures in the afternoon getting nothing!

Notably, Southbound services don't get the 'full' dinner, although the difference is slight. (More choice in the hot dish, and a more extravagant desert.)

Don't get your hopes up too high. It's pleasant, but it's not fine dining and the portions are not overly generous. Service works to a rigid and often incomprehensible timetable (you'd better take the coffee when it's offered because there's no way you're getting a cup with your slice of cake later if it's not on the official order of service). Expect to have your table cleared and relaid for the next service well before you're in London.

That said, it's a very pleasant way to travel, particularly if you get a good crew who are liberal with the vino (it seems to be the Newcastle-based crews who are the best, but perhaps I'm just biased). I pay the extra for the quiet and space if it's not outrageously expensive. I see the food and drink as a nice perk, but it's not my reason for paying for First.