@surreybloke add a neon screwdriver to you test kit, easy check of the polarity.
For an isolator I would just use a 2-pole MCB in a plastic box (get here no problem), choose the current rating to suit the meter 20A for a 5-15, 50A for a 15-45, 100A for a 30-100.
You should be able to find a local "sparks" who will install it for you either live or by pulling the tails at the meter (Meter seal? What meter seal?).
Once you have that you can pull the breaker and completely isolate the system and work safely.