I am wanting to visit 78 Derngate (Charles Rennie Mackintosh fan here) in Northampton. It appears that is an easily done day trip out of London Euston Station.
When looking at the map, it appears that Bletchley Park is on the way! How do I arrange my ticket to allow me to jump off for a few hours, tour Bletchley, get back on and resume my journey? Must I book 2 separate tickets - Euston to Bletchley & Bletchley to Northampton?
I don't know enough to answer your actual question, but I did want to say that arranging a stop at Bletchley Park will be well worth your time. It is just a short walk (less than 5 minutes) to the visitor center from the train station and the tour is extremely interesting. I'd allow at least 2 hours for the stop. They do have a very nice cafeteria onsite if you are there around meal time.
As far as I understand, Advance Tickets do not allow a break of journey, though Off Preak and Anytime Tickets generally do allow break of journey.
Scroll down the near the bottom of the page "A simple guide to train fares..." https://www.seat61.com/train-travel-in-britain.htm , that'll tell you which ticket types will allow breaks of journey and when.
https://www.nationalrail.co.uk/times_fares/ticket_types/CDR.aspx#journey
https://www.nationalrail.co.uk/times_fares/ticket_types/46590.aspx
https://www.nationalrail.co.uk/times_fares/ticket_types/anytime-tickets.aspx
https://www.nationalrail.co.uk/times_fares/ticket_types/46546.aspx
A Day Return from London to Northampton is what you want. Off-Peak if your outward travel is after about 9.30; Anytime otherwise. These allow a break of journey. Take any train.
No need to book ahead just buy on the day - same price.
I spent nearly the entire day at Bletchley Park. It still might be a reasonable combo, since I assume you won't need too long to see the Mackintosh house in Northampton. I do recommend an early start, though.
Thanks for mentioning 78 Derngate, by the way. I've added it to my notes for a follow-up trip to England. I'm annoyed I didn't know about it at the time I spent a night or two in Coventry.
