Check for the "thatchem" approved stickers on any well known makes., but it depends on if you are carting the lock to work and back or if you can simply leave it there. Strength means weight, a lot of weight. Also check E Bay for used M/cycle locks, [if you can get a £100 lock for £50 why not ?]. One last tip if you use a chain, try to make sure the chain can't be laid on the ground when in use, tricky I know but worth trying to find something high up to chain the bike to. Almost any chain, good or bad can be cut with croppers if they can be braced on the ground and then jumped on, if the theives have to use arms only then the power of the cut is much reduced.