Storage is a big industry here, people having so much junk and so many garages.
Super-large boxes become a considerable pain once you start wanting to stack things or put them on Ikea shelves or whatever. See-thru plastic is good, so you can see the stuff you want. Generally they aren't, not exacly fragile, but not burly either. Standard blue boxes come in two types 1) as cheaper harder plastic which is slightly brittle, but in any case will crack, and 2) the more robust, slightly deformable, slightly rubber-feeling plastic which is virtually indestructible. RubberMaid is good at about $10 each. We have lots of that.
I have one box which is tough as anything, made of hard plastic with a very positive closing, which I put in the back of the car for camping organization and, if camping near the car, use it is a bench. ActionPacker - it cost about $15 I think. It is great
I don't have anything watertight, so on the rare occasion that anything susceptible to getting damp or picking up an odour goes into the garage (which carries the lovely scent of racoons), I put it in a bin bag with a few of those silica gel packets - which I have been collecting over the years and have any number of now
Anyway, whatever the UK equivalent of Canadian Tire or HomeDepot is the place to go