Not exactly a direct response to your questions, but for me, Tymit was a great option as a back up card.
I have a Nationwide Select (very large credit limit) which is used for high value purchases, mainly for the section 75 protection.
As a back up, I originally had a Nat West credit card (small limit, but adequate as a back up) which was cancelled by Nat West due to inactivity.
This was replaced with a Tandem cashback card which unfortunately is no more.
With the demise of Tandem, next was an Aqua cashback card, who unfortunately kept pestering me to use the card and so were told where to stick it
This brought me to Tymit, which has a decent credit limit for a back up card, the 3 month interest free option and no hassles if the card isn’t used for a while.
So far, for my needs, it has worked out perfectly.