Britain has drydocks which can cope with hull lengths of about 900ft, which is why the Malta class, CVA01 & 02 and the CVFs are all about the same 900ft-ish length. Any longer and the already staggering cost of new carriers is driven up by the need to enlarge drydocks to cope with the extra size. I think that even the US is up against this limitation now at 1100ft-ish , which is why the Nimitz class is getting fatter but not longer.
Not that Ark follow-ons would come up against this limitation, that was the job of the Malta class. However if the RN did go improved Arks then they may have built the Malta class, even if it was postwar, because they would quickly see the value of large airwings and pulsed strikes. The Malta did have an armoured deck, but not an armoured hangar so postwar mods wouldn't have been so difficult/expensive as the Victorious/Eagle/Ark Royal-07.