best way to travel in uk for 17 days

It looks like you can get a discount on the tube (off-peak only), using an Oyster card, if you "add" your national rail card to it. Either the senior rail card or, maybe, the Two Together. May be more trouble than it's worth, but here is what I found:
https://tfl.gov.uk/fares/free-and-discounted-travel/national-railcard-discount

It depends how you want to travel- flexibly on a whim or on timed trains booked in advance.
If the former then Britrail is your friend.
If the latter then buy a Two Together Railcard and advance fares.
If you had a Senior Railcard you would get 1/3 off London travel when using Oyster (so the Oyster would pay for itself) but not with Two Together.

With a railcard some of these Advance Fares are so cheap- Cambridge to York in a few days time about £35 each, in a months time depending on the train about £20, York to Hexham about £10 (sometimes it pays to buy that as York to Newcastle, Newcastle to Hexham, sometimes as one ticket), Carlisle to Glasgow is widely available for £6 to £10 with a railcard.
To Fort William book it as return Glasgow to Fort William, and single Glasgow to Edinburgh.
The other way from Fort William to Edinburgh direct through totally different scenery on Ember Bus for £23.30.

Two Together Railcard (that's the one for couples where at least one person is over 60)

Two things here -

none of the conditions of a Two Together Railcard require any particular age or relationship. It is designed for any two people who travel together (hence the name) and don't split up to travel apart. It has time restrictions and required passport sized, but not necessarily that quality, of both named individuals.

Senior Railcards are for seniors and have fewer, different restrictions.

They are all explained in the Railcard link above.

Hi all,
Thanks for clearing up my confusion!

We bought the Two Together card, and will go contactless for tube travel.
Thank you for your speedy responses!
jeanne