Any ebike sold from within the UK officially must be fully certified. I guess perhaps you have Chinese sellers who set up a despatch warehouse in the UK might get around it somehow. I.e. the payment goes directly to China and they despatch from within the UK. Generally though the way to bypass cert tests is to buy from Chinese sellers on Amazon or ebay or buy from Aliexpress for goods directly shipped from China. In India and China there are a huge amount of ebikes many not properly certified and still ebike fires are very rare overall compared to how many are in use and in those countries ebikes get huge use, for some its their only means of transport. Admittedly in india they have a lot of ebikes using lead acid batteries and even brushed motors as that is what is available at the lowest pricing.
It's definitely of concern but lets face it you have a much higher ratio of car fires, Quite a few Ferrari's and Lamborghini's have caught fire and they sell in small numbers so the amount that catch fire must be quite high.
When New York did that image of the ebikes that had gone on fire. It looked more like one big e-motor bike, lots of small wheeled proprietary chinese ebikes but mostly hoverboard and scooter type devices. Not many regular size ebikes. Maybe with their thinner frames normally ebikes just turned to dust. I think aluminium burns at half the temperature of steel so I guess its more likely there would be nothing left of aluminium framed ebikes if the fire was really intense.
Also I noticed two of the ebikes shown first when I googled 'ebike fires' clearly show mid-drive motors. Which I thought was significant as mid-drives only make up a smaller amount of ebike motors sold and of that niche I would of thought store bought pre-built mid-drive motor ebikes were more common. Typically mid-drive requires more peak power from the battery to create their higher torque figures. I wonder if this could be partly responsible or maybe its just an indicator of the type of people that are power obsessed and go for the most powerful motors and don't use a battery to match. In my experience hub motors don't have the range of current requirements of mid-drive motors which have a greater current draw range.