There may be more and more people using the electronic cigarette, but there is still a huge number of people smoking cigarettes, cigars and roll-ups. The majority of matchstick sales are now in 'cooking matches', I doubt the electronic cigarette has dented the matchstick market, but cheap lighters are like any other consumer product. As long as worldwide sales are high of that product, they will still get shipped to the UK on the massive container ships and sold for next to nothing... it's capitalism with a small c.
Think about the amount of plastic tat you see filling the walls of pound shops, those 'gift' shops near every tourist attraction and in the odd supermarket aisle... they get shipped here in huge quantities every day. Shoving a few pallet loads of disposable lighters in there is no problem at all.
What has been effected is the consumables for more expensive lighters. You used to be able to service a petrol lighter quite cheaply from your local tobacconist, but recently the shops seem to be sticking up the price of wicks and flints. It's cheaper sometimes to buy a pack of cheap disposables and strip them for their flints... but then that is kind of weird because you thrown away a perfectly good set of lighters for the sake of getting consumables cheaper.
A side note, I smoke and I smoke a fair amount... personal choice and like quite a few others I've spoken to recently about smoking, the whole anti-smoking thing, the push towards 'helping' people give up has been exposed by electronic cigarettes. It was fine when the big drug companies were charging a fortune for patches, sprays and gums... but if the government AND the pharmaceuticals start to lose money by the sale of something that really does help people give up, look at them scurry and hand-wring about new legislation.
It was never about public health. It was about dividing communities (a divided community tends to do as its told more readily) and it was about replacing the profits from tobacco sales with sales in pharmaceutical products. Unfortunately the advent of the electronic cigarette has meant that people can smoke them indoors (hence no more division) and no more huge profits for overinflated tobacco prices and overinflated pharmaceuticals.
So yeah... cheap lighters