For London to Bletchley Park there are technically cheaper Advance Fares, but for that kind of journey that is just adding a complication to me, in that you are then bound to certain trains.
As well as being high tech in buying tickets on an app when already at Euston, you can buy paper tickets on the ticket machines, for the same price.
Also there is now contactless payment to Bletchley Park- no good if you have a National Railcard and/or are wanting to use the 2-4-1 offer at Bletchley Park.
If I've gone to the effort and expense of going somewhere such as Stonehenge I'm spending a lot more than 5 minutes there. If I had that little interest I wouldn't be going there in the first place.
Having said that it is surprising how much Stonehenge is a must do for overseas visitors, not for Brits. I was at Salisbury for almost a week last Autumn. Stonehenge wasn't even on my agenda, and no one I know even thought I would be going there- people could rattle off a wholly different list of other day trips (as could I)!!
I'm not saying I wouldn't go, it is just way down my Agenda for that area.
Horses for courses.