Do all modern boilers require 22mm gas supply pipes, or are there (combi) boilers that can be fitted without lifting all the floorboards to replace the existing 18mm pipes?
Depends on power of boiler, length of run from meter to boiler, number of bends, and whether other appliances are also connected. Sometimes you can get away with 15mm (not 18mm, that doesn't exist in the UK), sometimes you need 22, 28, or even 35mm.