London is expensive, but it does vary completely according to the type of restaurant you choose. Chains like TGI Fridays expect to pay say £15+ for ribs, and comparable chicken entrees, and I echo the last poster where they said re-fills on sodas are not the norm in the UK so that can bring the amount up. I do believe that Pizza Hut are the exception to this. We have a range of reasonable pizza places Pizza Express, Ask Pizza as well as Pizza Hut and their prices are roughly £8-10 per pizza for one person. We do have some carverys in the big London Hotels ( eg Strand Palace etc) where you can sample an all you can eat typical roast with a variety of meats,and vegetables etc but they do cost more, but may be worth it for the experience. There used to be a good one in The Tower Hotel which overlooks Tower Bridge so very historic. We combined going there once with getting tickets to The Ceremony Of The Keys which is a free evening ticket only event at The Tower Of London which must be booked in advance, but is a unique experience. You get to go in the Tower after it is closed and watch the 'keys' being handed over for the night by the guardsmen. You can of course eat much cheaper at some of the oriental buffets around Leicester Square, or you can go the other end and eat at some of the many upscale places, but be prepared to pay £££££££££ You can take children into many english pubs like Weatherspoons where you can dine cheaply and there is one of those near the tower too.
Just don't be tempted by places like The Angus Steakhouse or any other similar, because in my experience they are dreadful. High prices and bad bad quality.