Why do you ask if you know the answers? If you don't see CGNAT as a deal breaker then that's obviously a legitimate opinion to have, but make the case on its own merits, don't just ask other people to tell you why CGNAT is bad.
90% of the UK isn't CGNAT, it's probably the other way around by connection quantity. All the big longstanding providers like BT, TalkTalk, Vodafone, Virgin, Sky have massive IPv4 allocations so don't need to go near CGNAT. It's the altnets that tend to default to CGNAT to make more efficient use of their ranges. Some of these altnets do CGNAT and no IPv6, which I would avoid.