If started from turf, as soon as the pieces of turf are well stuck down. If started from seed, as soon as the seedlings spring up after being stepped on. Also be sure not to take off more than 1/3 of the foliage height.

If you are in an area of the UK that is experiencing a drought I'd be careful of cutting at all right now. If you have a neighbour who is a keen gardener with a lawn you could ask their advice.