Suggseys quote is from another policy, as you have found M&S have no small print specific to bikes. I’m sure there will be something in there along the lines ‘taking reasonable steps to secure your property’ but they arn’t going to turn round and claim your £100 lock isn’t sold secure gold, or that your shed has the wrong type of lock.
When I added my named bikes on renewal it didn’t make enough of a difference to spur me into shopping around.
My bike is secondhand and most of the parts have been on various bikes over the last five years. But the policy is new for old, so I just found the current model with vaguely similar spec (race spec, apparently!) and gave them the rather impressive RRP. It would probably sell now for 10% of that, but anyway…