Anyone used a GHIC card

[Snooky]

Shortly after when my autorenewal should have been I checked and they had cancelled my auto-renewal

Unless they notified you in advance via one of the methods you agreed to, this sounds very wrong, considering their default when you buy annual insurance is auto renewal and you have to explicitly opt out of it. It's completely reasonable to assume they'll do what you agreed with them.

In your position I'd make a complaint to Admiral and if I wasn't satisfied with their answer, ask them for their final response and then raise a complaint with the Financial Ombudsman. That'll cost them about £650 regardless of outcome so they'll probably offer you some compensation to make you go away.