If you are a visitor to London and you want to have a cheap meal but also have the most spectacular view I would like to suggest that you visit the Strada Restaurant at More London, SE1. It is right on the riverside and has views down river to Tower Bridge and up river to London Bridge and the HMS Belfast. Great views of London are not unusual in London. The Shard (very expensive), the OXO Tower (very over-rated in my experience) and the newer 20 Fenchurch Street (also known as The Walkie Talkie Building) offer incredible views but they all all pricey places to eat. Strada is a chain of Italian restaurants that is fairly cheap (especially as they often have discount vouchers on their website) and the food, whilst not being gourmet is good with a large selection. The waiters at this Strada are very friendly and service is excellent. The way the restaurant is situated means that most people have a great view but to ensure this always ask for a window seat and the upstairs floor view is slightly better than their ground floor. A really great place to eat.
Tip: Register on the Strada website and they will send you a voucher for a bottle of Prosecco for your birthday (when you spend £30 on food). Another reason to visit!