Is this bay tree in a pot or the ground? And how long have you had it?

This is normal after a hard winter. Parts of my tree look bad too. But the good news is that new leaves will be produced soon to replace all of last years leaves. Evergreens tend to do the 'autumn' thing in the Spring. Take the opportunity to trim the tree to shape and cut out any obviously dead shoots, if needed, and wait for new growth.

The tree (bush) is in the ground (been there at least 15 years) and is approx 6ft round. It has always been green and hasn't gone brown like this before ?

It will have been badly affected by the sudden cold temperatures during February after such a mild winter, but the markings on the leaves are interesting. Check the bark and stems and backs of leaves for invaders - scale insect might be an issue. How much of the plant has gone brown? Is there new growth showing?