Like with many things in life, what level of security and the associated cost you throw at securing your bike is probably likely to be a compromise between how much the bike is worth, either in money terms or to you and also where you are going to be leaving it and for how long.
It’s always worth remembering that no lock is unbreakable and given enough time a thief can defeat it, so it is worth getting some insurance.
Two things worth noting about insuring your bike...
It usually doesn't cost much more to add bikes (if not already covered) to contents insurance if you have it, and that many of the better locks on sale today come with insurance sometimes up to £1500 if it is defeated by a thief, so check out all options.