So if your bike if worth £600+ you'll get the full £350 if it's stolen - usually!
Some house insurance does cover the full value of any bike.
Otherwise buy a bike specific policy. I did that but it cost me £80 for 1 year to insure 2 bikes with total value of £1500
I forgot to say: bike specific policies are most useful and straight forward if the bike is new(ish) and not modified much - ie. an off the shelf bike. Some insurers will insure custom built bikes but you'll pay through the nose, and will require a receipt for every component.