You have to make sure the circuit can take the extra load and the earthing arrangements are ok, you become responsible for the work you actually do, which as I said obviously means making sure it can support that work
You do not become responsible for the whole circuit... how can you, you didn't install it!
You also have a responibility to flag up and note any departures you find regardless of what circuit they are on,
so if you notice the ring has a chain of spurs off it... you have to flag that up regardless of whether you are working on the ring or not, for example
What you propose is just a surefire way of making sure that if the ring circuit has faults that may be only found by testing, that they stay unknown about...
(although I would recommend that boilers go on non rcd wherever possible rather than an FCU on the rfc, but thats aside from the point i'm making)
P.S. The reg calling for a single point of isolation is often mis-interpretted (well it must be, I keep hearing it interpretted two different ways... one much be wrong

), but however you interpret it it does not have to preclude you having two consumer units, if you cannot see how you can have two consumer units and still have a single point of isolation then I'm not sure how much of a good idea it is you doing this work anyway...