The adult one is going up from ?51 (I think) to ?66 and the child's one from ?34 to ?45!
Martin's money tips (moneysavingexpert.com) says that they can carry over 9 months validity to your new passport. It makes sense therefore to renew now if you have 9 months or less to go on your current passport.
Just the delightful passport photos to organise!