I buy a bit in from Hong Kong, although not heaters, but you can bet that if they're selling direct to the UK they, or their supplier is filling containers and selling to wholesalers here anyway, so if its CE marked or similar I don't see a problem.
Before you buy check their feedback, and see how they have dealt with any faulty units and returns, import duty is a pain in the harris if you get clobbered for it, but (I didn't say this), most Hong Kong sellers undervalue stuff on customs forms, I just ordered in some RC car kits for resale, in the UK they retail at £399, wholesale at £312 with the VAT, and were advertised at £200 with free shipping, I ordered them in separately, and they arrived separately, all were declared at $50 US value, so nothing to pay there.
If the deal goes t1ts up and the unit is under 2kg, chuck it back to them airmail signed for at around 20 quid, take that bit of it on the nose and get most of your money back, you can't really lose.
As for if the unit went bang, if you purchase UK supplied kit and it went bang then legal responsibility goes back to the importer, if they don't decide to go bust or change names, as plenty do regularly, the disclaimer is in the text on the listing "should be installed by professional gasman", if you aren't corgi (or whatever they call it these days) registered, and you're fitting it yourself, then there is no claim, whether the unit was supplied by a UK based company, or imported.