Nick wrote:

The supply to existing boiler is 1/2" steel pipe. I'm told
by who?

that I need to have a 22mm supply for a new condensing boiler.

It might be true, or it might be a lazy installer not wanting to do the
sums ... British Gas tried that trick when my father had a new boiler
fitted, it didn't wash with my Dad who used to work for British Gas* and
did the sums for them!

Would it be legal to run the majority of the pipework in armoured
rubber/flexible pipe?

Rubber no, but you could suggest corrugated stainless steel tracpipe
[*] misplaced loyalty means he still expects them to be "good".