Hi, 

That is called White Prunicola Scale (look under the scale insects tab) and it is a common pest of Cherry Laurel. The link will guide you on treatment but with the high population like you have you should prune out the dead and heavily infected branches.  The rest of the year you can treat with an insecticidal soap or horticultural oil during the crawler emergence periods to help smother the nymphs. Once they take on the adult form a grow a protective coat it is difficult to treat for them and better to do manual removal like scrubbing them off the branches or pruning out of the shrubs. 

Let us know if you have further questions with treatment. 

Emily