There is no way that will meet wiring regulations. The requirement for an RCD is mandatory unless you can run the new circuit wiring in armoured cable or earthed metal conduit. Or keep all the new wiring at least 50mm deep in walls. Otherwise you will need an RCD in that circuit.
Let's be clear about that use of "mandatory." It is mandatory in order to comply with the current edition of BS7671, but as compliance with BS7671 itself is not
compulsory, it is thus not mandatory in the legal sense, which is how many laymen would interpret the term.
More generally, you should consider looking at a consumer unit upgrade to include at least two RCDs.
Don't you think that completely redesigning the house's distribution system is a little excessive just to add one light? Maybe there are other reasons why one might want to change something else, but there seems to be a growing tendency to suggest that everything upstream of any proposed wiring change or extension needs to be brought to the current standard.
You are saying just RCD protect the new circuit from the FCU onwards. I don't like it because it is adding a new circuit to a non-compliant system but I suppose that might obey the spirit of the 17th edition
It's not adding a new circuit, it's extending an existing one, and even when complying with BS7671 for the new addition there's no requirement to add RCD protection to the existing parts of the circuit. The RCD-protected FCU for the new wiring would be fully compliant with BS7671 (subject to no other problems with the existing circuit, such as missing earths etc. already mentioned).
But and its a big but, you will be extending the circuit of a non-compliant installation. That is, your existing electrical installation does not meet latest wiring regulations.
There is no requirement for any existing installation to be brought up (or down) to current standards.
PS, How non-compliant is not known but the fact that you have wired fuses is a clue that it may have problems lurking due to age and may have been poorly DIYed over the years.
True, but equally it doesn't mean that there definitely are
problems, just as a shiny new board and installation installed when the 17th edition came out doesn't guarantee that somebody hasn't been along and messed up something in the last three years. Without checking, it's impossible to say.
Chances are there is no main protective bonding in this property.
What makes you say that?