L.flavus, or commonly known as the Yellow meadow ant.  Often mistaken for red ants - M.ruginodis due to their colour these little fellows are one of the commonest ants in Britain, yet they are not seen very often...Why? Because these ants forage for their food under the ground.  You will have definately seen them as they build the very large mounds of earth.  And the reason for that is that they dig masses of tunnels underground to search for their food, so the soil has to go somewhere.  It is also an important way of keeping different temperature variations within the nest.  A large mound of earth is so much easier to keep climate controlled than a small area.

They eat just about anything, and are pretty much the same as Black garden ants - L.Niger in most other aspects.  They also have nuptial flights that occur once or twice a year, and only have the one queen per nest. They even farm aphids for nectar, even though the aphids are are also kept underground. 

Yellow meadow ants range throughout britain, but mainly in the South of England and will happily live in quite diverse temperatures. Mid 20's centigrade in the summer, and a cooler 15 centigrade in the winter would be ideal so that they can overwinter.