No I'm not going to ask for recommendations on a "cheap" place to stay in London. But....
I'm looking for accommodation in London and nearly every hotel lists "Twin" and "Single" rates. What is the difference?

Marion_Ravenwood
1
sneaker_fish
Put simply, a single has one bed, normally a single/one person bed. Your rate would therefore be for just one person.
A twin tends to be two beds. Quite frequently, this means two single/one person beds.
You didn’t ask but the other option you may see is “double”. This generally means a double bed. Sometimes it is available for single occupancy, but generally assume it is for two people unless otherwise specified.
Pro tip