What I find quite strange is the whole debate on here about what in effect is plain simple English. Debate that is as irrelevant and boring as the number of angels on a pin head. The Law in its interpretation of the detail tries to use the embodiment of the "man on the Clapham omnibus", e.g what an ordinary person would deduce. Hence the simplicity of the requirement to test before working on a circuit, and immediately afterwards.
My lifetime experience of the debates that end up trying to argue that "No actually means Yes" are all based upon someone wanting to worm out of doing something that should be done, and you would not believe some of the convoluted arguments to be had on when is a lifting operation a lifting operation, etc, etc.
Sorry if this offends but the process is that you need to undertake sufficient prior testing as is reasonable to ensure that you can actually carry out the work without something unexpected appearing. Whilst there will always be that potential for surprise, you should go into the job as best informed as possible and not end up unable to re-connect because of having identified part way through something that a little pre-inspection would have identified.
I would always explain to a customer that it is necessary to ensure that the work can be carried out or that if not then the customer can be given a clear indication of what the alternatives may be and at what price these alternatives may be achieved.
This should also remove the ongoing thorny problem of the customer feeling that the electrician is taking the P having started a job that cannot be finished in the budget price. Oh and by the way a quotation is exactly that. You find something out part way that means for example a re-wire, and there is no legal obligation on the customer to pay unless it is a demonstrably demonstrated that the matter could not reasonably have been foreseen by a skilled and competent person...and the more experienced you are the higher this bar !