A double socket is only rated to carry 19.5A continuously (MK). A BS1363 plug suffers from 8 watt heating of the fuse, plus a contribution from terminals & fuse tabs. Many immersions can actually drift past 3.3kW. Most sparks will fail a BS1363 plug (top) feeding an immersion.
It looks like people are suggesting fitting a plug-socket because the immersion is only used in emergency, thus unplug the light and plug in the immersion?
You could do that with a BS4343 16A IP44 plug-socket.
- BS4343 are unshuttered, so really need to be in Switch Interlocked form (plug can not be removed without power being first turned off, thus compliant with domestic requirements and avoiding flash if removed on load anyway)
- Gewiss do horizontal, vertical & compact Switch Interlocked for about =A312-14 from Discount Electrical. I have never used them or used the product, but I've kept a mental note of "compact interlocked one not a bad idea for garden extension lead if the SWA damaged again". Shipping will sting you for =A38 though - still cheaper than a Masterseal (still the best outdoor).
You can use that for DIY use upstairs since it's a dedicated radial whilst doing maintenance on other circuits avoiding running extensions down stairwells and such like, so perhaps "more justification" for the DIYer whilst legal :-)
I would not even use an MK B1363/A plug and masterseal socket for an immersion. Even 1.5mm flex gets "pretty warm" running 3kW for long which contributes to the terminal heating re wire size, contact area, fuse tab size, fuse dissipation etc. It all adds up. If you had silicone lead to the immersion and BS7211 fixed wiring supply (90oC) then "ok". Perhaps overly cautious, but immersions used in emergencies can see continuous duty and plugs are not good at 3.3kW - my own just before failure a few months later pulled 3620W on a clamp meter, they can drift. Some immersions are wired in 1.5mm which in PVC isn't going to like the terminal heating from the plug - it's lack of heatsink area. 2G has more metal to heatsink with and moves the terminals further away from the plug. Airing cupboards can get warm though (40oC