You can't organise a cathedral wedding at "short" notice - I reckon it was always going to be there! after all, with Andys family, Kims family, their friends they are wanting to invite, I suspect it was always there.
Dunblane actually surprised me as a venue as it's usually the bride's home town (so that would have been London area) that you go with (eg - My husband is from Kent, I'm originally from Yorkshire, although we live in Kent, we married in Leeds) - Banns have to be read in BOTH parishes (at least it used to have to be lol) so this could still all be a red herring
Andy did say it would be up to Kim to decide, but like you I never really believed they'd get married in the tiny chapel at Cromlix. Then when he mentioned the other day about best men and bridesmaids it was obvious it wasn't going to be there. I must admit though that the cathedral came as bit of a surprise!