You can install a RCD, however that does not mean it will work. The pre-testing is to find existing faults that will stop the RCD from switching on.
Over the years most houses have had alterations and many not done following even the current regulations in force when done. Years ago we for example had a 5 amp supply to the lights, and in the late 80's and early 90's people when mad fitting little spot lights which increased the load to over 5 amp, so they would split the supply up stairs and down stairs, however this should not really have been done as the landing lights borrowed a line. However it worked so no one worried too much, it is called a borrowed neutral, and is against the rules and as far as I know has never been permitted, but fit a RCD and ups you can't reset it. So now must be corrected.
My house has both sockets and lights split into two, the lights split up/down the sockets split front back, the clause "Every installation shall be divided into circuits, as necessary, to take account of danger that may arise from the failure of a single circuit such as a lighting circuit." means sockets and lights in any one room should not trip the same RCD, so it will need at least 4 RCD's in the consumer unit for my house, since consumer units not made that way, with my house I must fit RCBO's to all circuits to comply. Clearly you want to know that before you buy the consumer unit.
You could install a RCD in each circuit after it leaves the consumer unit, you could fit many consumer units, there are many ways to comply, however in the main any unusual method will cost more.
So an electrician could swap your consumer unit for an all RCBO consumer unit without testing until installed and simply lock off any circuit that does not comply when complete. However when he says that's £750 please oh and by the way you have got not lights, would you like to book a date for me to fix them. I think you may be a tad upset?