It was inevitable that they went really. I used to use them for all my electronic hobby needs when I was young. The catalogue was a repository for everything electronic, which included detailed circuit diagrams and component specifications. The prices were sensible and the mail order system was excellent, although a little slow.
In the early 2000s they moved away from the electronic hobbyist and went into general tech instead. This was a bad move imho, as many hobbyists, me included, simply stopped using them. I loved visiting our new local Maplin superstore, but only bought stuff if I was desperate for it. Otherwise I simply looked at what I wanted, then ordered the identical item for a quarter of the price elsewhere.
I don't think brexit has much to do with it myself. I'm more inclined to blame the internet and sites like EBay. I buy loads of cheap electronic gear off there. It mostly comes from China at a fraction of the cost that Maplin could sell it for. It's expensive having large superstores and lots of staff to man then. These online warehouses have little in the way of overheads, so can sell and ship items for a fraction of the cost if a UK retainer.