Not sure I quite envisage the setup here - Is it that tails run direct from the meter to a small CU containing an isolator and two breakers?
Yes thats right.
If so, the correct approach would be to tap into the shower cable itself downstream of the breaker, and fit an RCD into a 2-module or 4-module DIN enclosure. These are only a few pounds.
If an RCD is mid-stream then how can I test the installation to varify the installation integraty? How do I get 500vac through an RCD mid-stream?
If OTOH you want to disconnect tails from a meter, or from Henley blocks on the supply side you MUST contact the supply company first. Otherwise they are likely to suspect you of tampering with the meter, in which case they may get very shirty with you! The correct, safe way to do this is to remove the supply fuse from its sealed holder (again you MUST contact the supply company)
Some electricians may work 'live' on Henley blocks, but bear in mind they have a great deal more experience than you, working live on a 100A circuit is most definitely not for beginners. The issue here isn't so much shock, but an accidental short which could leave you with burns or even eye damage from the flash. Basically... if you're at all unsure leave this kind of thing to the experts. Don't intend putting myself in hospital, I have had arc-eye off a welder before so could guess at what 60-100amp's would give me!
Might also add that it's not mandatory to have an RCD ona shower feed. Personally I like to see one, but many electricians don't fit one. The main point is to ensure that the shower and its feed pipe (if metal) are well-earthed (use 6mm earth wire) and that all touchable metal within the vicinity of the shower (bath-taps etc) is cross-bonded together and linked to the shower earth. Principle here is to ensure that a fault in the shower cannot create a voltage between adjacent metal fittings.
I am well familiar with supplementary bonding of supplies.