There is a national budgetary policy which can give financial support to regions which are lagging behind the general trend. And there is a largely similar and similarly funded system of welfare. These are all things which the euro area does not have at a supranational level — or, at least, not to anything like the same extent.
There are some — mainly on the continent — who regret this because it means that economic convergence cannot be created or, at least, actively promoted by a centralised policy. Others — and I suspect that the overwhelming majority of the British people belong to this camp — believe this to be a good thing.