Single pole or dual pole relay for switching a 3KW water heater? (UK)
That item probably should not connect to AC mains

Each country has apparatus to protect its citizen from dangerous products.  Most countries require good be certified by a competent independent testing lab such as UL, CSA, BSI, TUV etc, and retail stores can get in big trouble for carrying uncertified items.  The EU allows companies to self-certify, but that only applies for businesses physically in the EU who ship from the EU to the EU.  And shipping via dropshippers like Amazon Warehouse does not count!  

The CE mark (that self-certification mark) is worthless for things shipped from Amazon or outside the EU.

Certifying testing labs isn't really in USA OSHA's wheelhouse, but they've done a good job so everybody defers to them. Look for the mark of an NRTL. 

From its configuration it's a USA/world market item.  The packaging/literature lied about its internal abilities.  It's a no-name or a brand you didn't think important enough to mention.  

So this rings similar to what we see a lot of, which is that cheap Cheese junk brought in mail-order.  Search for a mark from an NRTL (CE isn't it). If you see ETL or TUV or UL, it's safe to let it touch mains power.  Otherwise, heck no - keep it on the low voltage side of the certified wall wart. Have it use low voltage to operate a certified relay. 

Components have their own certification system; one common one us RU (with a Russian R for some reason). That is UL's component listing. Ideally you'll see a mark like that on every component capable of taking a mark, and certainly the relay.

You're right, the relay isn't strong enough

Really, you want a margin of safety anyway, even with certified stuff. 

I recommend you go a somewhat different way. Rather than having the "smart WiFi device" switch the power directly, have it switch a (certified quality) relay/contactor which in turn switches the power.  If it were America I would recommend a Rib or Aube, which operate on 12-24V and fit right up into our style of junction boxes.  I don't know what equivalent equipment you have for mains switching in the UK/EU market, but I'm sure there's something.