If it's a newly planted bush this could well be the case as it is under stress. Bushes and trees planted now should be watered very heavily (a bucket or so) and then left for a week and watered again. Frequent light watering won't encourage it to put down deep roots.
If however it is a well-established plant the hard leathery leaves should be able to stand up to the British climate no matter how warm it is.

I've given it a good water and within 2 hours it's back to normal. the tree is 11 years old, I bought it 2 weeks ago and planted it in a big pot. so you suggest to water it a bucket or so once a week??

If it's in a pot it will need regular watering in very hot weather (isn't Britain supposed to hit 80F today?) Be careful not to overwater in cool dull weather as a bay is a tree from the Mediterranean area. If it's in a big pot, it will take a month or two to make roots into all the compost in the pot, and in the meantime its roots will still be drawing moisture out of the smaller root ball. Once established it should stand drought very well.
I suggested the 'bucketful' for one planted in the soil where it would drain away, so in a pot I'd give it a drench once a week in hot weather, and far less often when it's cool. Alternatively, stand the pot in a tray of water for a couple of hours once a week as the water will soak into the lower parts of the pot where the plant can use it. Watering from the top is often misleading. A clay pot is easier to regulate as you can tell when it's wet near the bottom by knocking it. In a plastic pot it's not so easy. And don't forget that far more plants are killed by overwatering than the opposite.

Hi,
A little tip for Bay's, they love a little dose of Epsom Salts in the Spring, just a spoonful in your bucket of water will do the trick! I also find that a liquid seaweed feed works wonders. :o)

Thanks for the advive