Can you give us some more information on your layout here?

CU approx 2.5metres away from Hot tub.


Why can you not use the "RCD Consumer Unit" as an isolator?

Because the CU also supplies lights & patio power sockets


Is it actually isolation you need or functional switching?

Isolation for the hot tub only


I'd guess it's in the installation instructions - and the 25 amp bit a
direct quote from the 'translation'.


Which raises the the possibility that this appliance might not even be
for
230Vac...

It is an English version hot tub of 230 volts.


CAUTION!!!!

3KW happens to be 25A at 120V...... Is this a USA appliance??????????
Please check the rating label!!!!!!

You're right to mention it, but hot tubs I've seen in the U.S. all connect
to the 220V supply. Normally 110v circuits are 15A or sometimes 20A..
Larger appliances are run from 220v to keep the current and gauge of wire
reasonable.