The heater takes things quite close to the limit 12 amps - so not much leeway, using a 16 amp socket and heavy duty flex makes it safer than the standard stuff, your ring main will take it, however if it's on the same ring as kettles, TVs etc, you run a risk.
To my (limited)knowledge items like ovens and immersion heaters are run directly to a fuse (rcd) board
Darran