I'm quite sure this is a global phenomenon, not unique to the UK
People are all about getting into non-committal relationships, and I guess that extends to a non-contractual relationship with a telco. Combined with number portability, there's no point to sticking with a telco or sign a long-term contract with it.
Most telcos also do not go all out to retain existing customers.
It's not really about 'phone fatigue'. Most people still change their phones fairly often, preferably when the resale value is still high. If it were really 'phone fatigue', the first sign would be the shrinking of the phone accessories and case market. Doesn't seem to be so.