Update: Fridays trip completed using the travelcards purchased. No problems using them on either bus or underground. No one asked to see rail cards.
I am not sure how the cost would have compared with using contactless bearing in mind we used discounted off peak rail cards. We did about 6 short tube journeys, venturing into zone 3 once, we basically went in a big loop covering all the places we wanted to visit, and used the bus for a couple of short hops, which we would normally walk, but I was (am) suffering from a poorly knee which is on the mend, and wanted to minimise walking.
If we visited regularly then we would look at contactless or oyster. Thing with contactless is I wouldnt want to be walking around with my 'main' bank card in and out of my pocket at each gate line, too much likelyhood of loss (I am somewhat careless) so I would need to set up a sacrificial account, where the card loss would be less problematic. Replacing a lost travel card is £9 odd, annoying and stupid, but no ongoing repercussions.
There were signal problems on the Circle line just before our return, we had to do a bit of a detour in order to get back to Kings Cross in time, made it with about 15 mins to spare.
Off topic: We travelled with Hull trains from Beverley using First Class advance tickets. Journey both ways was comfortable and almost on time (5 mins late into Kings Cross), with proper cups of tea made in a pot with Yorkshire tea bags, very welcome on our return as we had intended stopping for a drink and a snack on our way back to Kings Cross, but Circle line problems meant we ran out of time.