A final update – yesterday I completed the Tube Challenge (a ‘Full Network Challenge’, or ‘FNC’, of all current 272 stations). I will do a write-up at tubechallenge.com but will comment here on the ticketing for my trip.
Before Alan White asks, my time was slightly slower than both of his but, as are his times, mine was rather faster than the only Guinness-validated world record for the new configuration of 272 stations.
I followed Alan’s advice and used a combination of Day Travelcard (loaded onto a Smartcard) and contactless. I don’t live in London so all this was first-time for me hence, I suppose, the original question all those months ago.
I started from Chesham so the original plan was to purchase a Day Travelcard from Amersham, which would allow travel throughout Z9-1. But from this thread, I had learned that although that would work from Amersham if travelling directly to London by the Met line, it would not work for the Chesham and Watford branches, even though it is advertised as being valid for Zones 7-9 also. In fact, although I can’t find it now, I did find a bit of small-print on Chiltern’s website which said something like “your Travelcard is also valid for Zones 7-9 but it will not operate the gates on the Chesham and Watford branches. This is for technical reasons and if you need to travel to those branches you should speak with a member of staff at the barrier there.”
So, in the end, I realised that Alan’s approach is indeed the best one. I used contactless from Amersham to my first tap-out point, which was North Harrow. It cost £2.20. I bought a Day Travelcard, stating when purchasing that it was to ‘start’ from Harrow-on-the-Hill, that being the first station within Z1-6 for which it was possible to buy a Day Travelcard from Chiltern if travelling on my route. That cost £21.50. I bought it on my phone and uploaded it to my Chiltern Smartcard using my phone. I used it for the first time, with trepidation, not at HotH but on a bus from North Harrow, and it worked perfectly, as it did throughout the day at all Tube stations in Z1-6 where I needed to tap, on all buses, the London Overground and the Elizabeth Line.
So £23.70 for the entire FNC and zero hassle. It will be a great shame if TfL do abandon the Day Travelcard, as has been suggested.
My thanks again to everyone who helped and I hope that, along with previous comments, the above can be taken as a confirmed way to ticket an FNC.