A lot depends on the type of grass you have.
Grasses usually either form bunches that gradually get wider by growing from the edges, or form "runners" with a long rhizome with leaves that hug the ground. Fescue is a common grass of the bunch type, Bermudagrass is a common runner grass. Your local grass could be something different.
If you have the bunch type, try to cut so that most mowing removes 1/3 of the height of the grass (with a 4-inch mower height, try to cut before grass gets to 6 inches).
Runner grasses are usually more forgiving, you can cut shorter.
You can bend the above rules a bit if weather requires it from time to time.
Jezza is one fellow on this forum that may be able to give better advice on UK grasses.