Self insurance is always a viable option, which many could benefit from, if their pet never or rarely needs expensive treatment. One way to do this would be to just put money in a savings account, which would earn interest.
I see that MyPetCard has a monthly fee that appears to be effectively £4.49 if you fund the account by monthly standing order 'to help save money and care for your pet with our great money saving benefits we only charge £3.99 a month plus a small loading fee of 50p per transaction'.
But it is true that there will be a (possibly substantial) lead in period where your savings won't cover the veterinary treatment that your pet needs.
However, I suppose it depends how useful the phone and video support is. A lot of the time, they're probably not going to prescribe anything without physically examining your pet and also often it will be obvious if they need urgent treatment, eg if injured. But if you tend to be a bit anxious about every cough or vomiting event, it might be good to talk this through with them without actually paying for a full consultation.
The 'access to lower price medication' looks interesting and could be worth the membership if their vets issue private prescriptions - it doesn't say that they do though. A lot of veterinary practices charge a lot for medication that can be bought cheaper online, but you need a prescription, which they also charge for - so if membership gives you access to this cheaper or more convenient to your own vets, that might be worth the membership fee alone, but I can't see that they actually offer this, so you'd have to confirm.