For charts my preference is Stanfords allweather charts
. They are full size charts, printed on plastic "paper" and are extremely durable. You can spread them out properly for route planning, then fold them as small as you want when you're using them. I put the folded charts into clear A3 ziplock folders to keep them tidy.
Another option is the new Admiralty Tough Charts
. These are really rugged, printed on very substantial A3 plastic sheets. The biggest drawback is that they are a series of chartlets, and are not as versatile as the Stanford charts. I've got the "Area 1: The Solent and Approaches" book, and on half of the pages the actual charts are hardly bigger than A4, with a big border and space for notes!
Unfortunately neither of these cover the whole of the UK, so they may not be an option. Worth a look though.
If you have to use real paper charts, then a reliable option is to cut them up and laminate the sections, but you're likely to be restricted to A4 unless you can find an oversize laminating machine. Cear A3 ziplock folders are helpful, but in my experience don't necessarily keep the contents dry.
Most of the "proper" map cases that I have seen are quite small, as they are usually designed for walkers who don't cover the ground quite as fast as we do. They are also of dubiouswaterproofness! Aquapac do a waterproof map bag
that might be worth a try.