PedalSure state that if a bike is worth under £1000 (which mine both are) You only require a Bronze Sold Secure. You can get a list on their site. I am contemplating getting a Hiplock.
https://www.hiplok.com/product/hiplok-fx-red-cable-lock/
https://www.hiplok.com/product/hiplok-pop-cyan/
These are around £19.99 and are worth buying and using, especially if something were to happen and in the event of a claim.
PedalSure as well have the line about an "immovable object" - however, this seems pretty standard that people would attach bikes to these structures. I would never leave my bike attached the car and go somewhere else for a long period of time - there are opportunists everywhere. I do not blame companies for not covering theft in this way, I am lucky I can fit the bike in the car - so if I had it on the rack and then had to go somewhere, I could remove from the rack and put both in the car and covered.
This seems to be the general rule amongst companies:
"
For bikes valued up to and including £1,000 we require that the bike is secured with a Sold Secure Bronze, Silver or Gold rated bicycle, motor scooter or motorbike lock. For bikes valued between £1,000 and £2,000 we require that the bike is secured with a Sold Secure Silver or Gold rated bicycle, motor scooter or motorbike lock, and for bikes valued at or over £2,000 we require that the bike is secured with a Sold Secure Gold rated bicycle, motor scooter or motorbike lock."
I am going to be purchasing a lock, most likely the Hiplok for summer cycling as I plan to do quite a few solo rides. It just gives me that extra mental security too!