I'm not an expert but a couple of mine have a few brown and crispy leaves and I've decided that it has been the strong/cold winds we have been getting here in Edinburgh for the last few days. I have one that is very sheltered and it isn't suffering at all.

Either strong winds, they hate wind, or lack of water if it is a newish tree and you are in the drought area of southern England.

Or could be a combination of sun and wind but wind is what they can't tolerate.

All I can confirm is Acers hate wind, they do very well if they are a bit sheltered, one of mine always does me proud as it is sheltered at the back by a wall about 3/4 of the size of the Acer. But with a combination of strong sunshine and wind, they are going to shrivel, its just like us really getting leathery skin if exposed to too much of these conditions