Maplin have been around since the 1970s when they were the go-to mail order store for British electronic hobbyists. But they're also having issues. Competition on bread and butter basics like cables from eBay and Amazon is killing them on price. Likewise PC World, who are struggling for similar reasons.
Argos and the like sell toasters, watches, cheap jewellery and gardening tools, so they're not in the same market.
Farnell (and RadioSpares, Mouser, and Digikey) are primarily industry suppliers who sell components to the tiny hobbyist market as a sideline, but make most of their money elsewhere.
The real competitors now are companies like Adafruit (US) and Cool Components (UK) who caught the Maker wave and are riding it very successfully - possibly because they sell clever, useful things, and they treat customers like adult engineers, not gullible children.
Radio Shack was already a legacy company by the 1970s, staffed by the kind of marketing geniuses who come up with ploddingly dreadful brand names like 'Realistic' and 'Flavoradios.'
The UK equivalent would have been Amstrad back in the 70s and 80s - purveyors of cheaply made over-marketed consumer electronics to buyers who were clueless about better options.
Clearly, RadioShack doesn't have much of a niche any more. Buyers can get better products more cheaply elsewhere, and the hands-on electronics business has moved in a direction their management doesn't have the talent to emulate.