Own Account Driver:
Unless you need for a specific contract I find 4.2m to be the best height means it will go under a lot of useful A road bridges like Wilton in Salisbury amongst quite a lot.Go over 4.5m and that tends to mean most marked low bridges will be no go.
I think bridges are the only down side of a high trailer but there are other routs that can be taken to avoid them
My concern is the restrictions i might have at customers sites if there will be any