My acers have already decided it's autumn and I would suspect yours may have, too.

I've no idea where you are in the UK, you haven't told us, nor have you said whether the plant is in a pot or the ground, or what kind of Acer it is, but if you're in the south, likely its suffering drought or the sun has fried the leaves. Water well.

Also, they don't do well in windy conditions. That may have been a factor as well as the unexpected hot dry weather.

The leaves on my new ones planted this year are brown, definitely due to strong winds and an open aspect, as I've not allowed them to dry out. Acers like a sheltered position afterall.