Bathroom water heaters

You could use a shower priority switch

That sounds like one option. Although 40+A change-over switches are neither small nor pretty, and probably isn't that convenient (too easy to forget to switch back after a shower, so no hot water elsewhere). An automatic  'shower priority switch' (a.k.a. a load shedding relay) is sometimes a neater solution if the supply isn't meaty enough for both (e.g. www.meteorelectrical.com/.../garo-1-row-shower-priority-metal-board.html )

Depending on the supply and other loads you might be OK just wiring both in normally (there are plenty of domestics with two or more instantaneous electric showers on a normal single phase supply which have given years of trouble free service, regardless of what the OSG might say about diversity).

Or these days of pressurized hot water cylinders, a simple immersion arrangement (especially one that can do the bulk of the heating from an off-peak supply) can be a quite workable solution, if you have the space.

   - Andy.

I can imagine a big switch outside the bathroom is going to be a pain in the neck for anybody using that bathroom.  the switch will always be in the wrong position and you will only find out when you try to wash your hands or get in the shower.

I much prefer the idea of a load shedding relay.

If the LSR1 (in that garo link) is a bit big, then it is possible to buy an MCB sized lump that is just the key part the actual load sensor and assemble it into any suitable enclosure.

https://www.eibmarkt.com/gb/products/Doepke-Load-shedding-relay-6-7-39A-RLR-1.html

Two terminals are wired  in series with the current to be detected, and the contacts that close can either be used to interrupt a small load directly, or of course more usefully  an arbitrarily  large load can be switched by  a suitable contactor.

Mike

Thank you all for great help 

If it's an instantaneous water heater over a basin, say, how likely is it that it would be used whilst the shower was in use?

If the basin only supplying a tap  the shower room?

Mike

Not that likely, but consider for example a nurse or care worker attending to a patient and washing their hands whilst patient uses the shower.

Or perhaps a maried couple who are not worried about privacy from each other., one showering and one washing hands/face in the bathroom.

It is unlikely that either device will be used for long, although they could be left on inadvertently.

Whilst small overloads of long duration are frowned upon (433.1), overloads of short duration are acceptable. A shower takes a few minutes. Washing one's hands in a basin, even less.