AF
I think you are right in your assumption that it is more easy to add an IMC/Night rating to your licence. But there are places Clacton which cater for tailwheel and stripflying. Other places are around but you will need to do a bit of research to find them.
May be the reason why this divide is there is because stripflying is a bit more challenging than flying a 152/172/PA28 and most people's budgets are limiting them in time available to spend flying.
I further think that the vast majority of strip machines are on a Permit or Private C of A rather than Public so that training is not possible even if instructors where that way inclined.
Training in the form of Circuits and bumps is not allowed at strips by the ANO, and would probably p$ss of the neighbours anyway, insurance companies are not keen on stripflying either.
See there a multitude of reasons why stripflying is less 'popular' than IMC/Night.
However that does not stop individuals to seek out that part of the flying spectrum and getting into a group/aircraft strip and enjoy themselves. It just takes a bit more effort and homework than pitching up at the local school.
FWIW
FD
PS: Poetpilot; I tried to contact them by emailing twice and leaving an answerphone message once and after 2,5 years still waiting for a reply - and yes, I am a PFA member.