Choose a household insurance that covers your bikes well rather than seperate bike insurance if you can. M&S are pretty much the default but try Co-op, AA and one I've forgotten the name of run by John Lewis.
Some household cover includes, or allows you to add a bike, but see what you are actually covered for, I had one policy once that listed the bikes but I later found out they were only covered while they were in the house, not while they were actually being used as bikes.
Specialist Bike cover often has lots of conditions – bikes only secured with locks from their list – not locked outside between certain hours at night etc.
M&S covers anything, doesn't care how you look after it of how/what/when something happens, and thats as much assurance as it is insurance.
