I just recently traveled to UK and France and agree the portions are about the same but I did not feel they were incredibly small. I felt most of the portions filled me up comfortably without left overs. I would typically not order a starter, a main and a dessert as many locals seem to do as that would be way too much food for me. I might order a starter and a main OR main and dessert or just a main. Maybe a glass of house wine as well.
During my trip I spent time in and purchased food in Bath, Conwy and London. I do NOT go for fine dining. I would say prices usually were in the 18-25£ range for dinner with one huge outlier in London. I went to a Thai place listed as vegan friendly and altho I looked at prices on the menu outside, the bill for spring rolls, veggie stir fry, plain rice and bottled water was 33£. (Came thru on the credit card bill as $55.) I was surprised but that was totally my fault as I had not added up the extras such as rice and water, lol. The food was fine but not worth the price. I felt I got better food at a neighborhood restaurant recommended by the hotel reception. I believe I paid around 20-25£ for a main and a glass of wine.
In Bath, I ate a couple of times at a wonderful vegan restaurant (The Green Rocket) and wound up paying around 12-15£ for a huge lunch which had me still pretty full at dinner so I just snacked. The higher amount was the day I also got a delicious vegan dessert. Oh wow.
Conwy, again a main and a glass of wine and it was perhaps 15£.
You may wind up with slightly higher prices than me, just because since I am vegan the menu selections are often limited so I get what I can. I also picked up some terrific to-go/takeaway prepared salads at some of the grocery chains. Usually they were in the neighborhood of 3£ each, so if you had 2 smaller salads plus water you are under 10£ for lunch.
I'm guessing you are calculating budget. I would say to overestimate your amounts and then you will be happy. Food prices feel high to me in Europe but I live in Idaho for heaven's sake and rarely eat out! However, you have to eat anywhere you are and if food is important to you, make yourself happy. If you are staying in hotels with full breakfasts and you eat those kinds of food you will probably not be too hungry for lunch!