No. They are trying to generate work. If they thought that this was the case to change it what happened to the site survey in the first place? Including making sure that all earth bonding is in place etc, before they proceeded. All they have to do is make sure that this new swa sub circuit is RCD protected, issue you with a certificate for that part of the work, this could be achieved easily enough without changing the main con unit. Ways and means. Unless it’s broken the main c/u does not need changing.

Its a recommendation

Plastic consumer units do not need to be replaced with new metal ones unless:
1) They are damaged.
2) You would prefer one as they contain a fire more effectively (and the consumer unit is the most likely place for a fire to start in electrical installation).
3) You'd like to make an addition or upgrade to your installation but;
a) There are no spaces for new circuits.
b) You cannot source the correct protective device (RCBO, MCB, RCD etc).
c) You don't want a second consumer unit (which it sounds like they've just done for your extension).

There may be other reasons I cannot think of. But in your case, it's up to you, there is no requirement to change it.

Agree with all above except one. The new sub-main does NOT need to be RCD protected. You don't RCD sub-mains. So the 45A mcb used in the old CU is perfectly okay.

Again agree with all but 1. If you had an rcd on the sub-mains your going to have potential problems with selectivity.