Andrew Gabriel wrote:

If you don't have the tools to measure the earth fault loop
impedance, you should assume it's too high to be on the safe side and
fit an RCD.

From the design point of view you can use Table 7.1 in the On-Site
Guide. If the circuit cable length doesn't exceed the maximum figure
given in the table, for the type and rating of fuse/MCB, the cable size
in use and the type of earthing, you can be reasonably confident that an
RCD won't be required. (TT installations excepted, of course.)

After installation you (somebody) must test the circuit and verify that
Zs is OK. (OSG Appendix 2 Table 2A, B, C or D as appropriate.)

What is important is to make sure all the supplementary bonding
in the bathroom is correctly fitted.

Absolutely. That's a much better step to take than unnecessarily
fitting an RCD, IMHO.

--
Andy