sharkbait – You are right in that they are precast – by design in other words rather than a defect and preventing rain sounds feasible. It might not ‘narrow’ – it might be that the precast sections are simply offset to create that shape – you can see it in the 2nd photo where the circular hole to the sky is partially blocked.
In effect it creates a bend that a liner would have to go around. The precast box sections may still be 7″ but to get a 6″ liner to bend sufficiently in such a limited ‘vertical’ distance seems unlikely.
Totalshell. As I understand it, a liner would heat up much quicker than a wider chimney thereby giving greater efficiency and all together less faff when lighting. Also restricts any nastiness regarding creosote formation (although I will attempt to burn the right things and not burn overnight).
Mrmonkfinger – DIY is limited to the first photo – everything else will be “expert” led. That said, some of the fitters that I have around have been shameful (suggestions of reg avoidance including one recommended strongly on here!!)