As far as the internals go, you can look up information on lever locks, the cruiser is a lever type lock with five lever mechanisms, to see how that type of lock functions.
Hopefully some of our members from the UK, or other members can help you more with that.
Your only real option is to impression a key for it. Since it's in the open position you'll have an easier time since you don't have any false gates going from open to locked, but if you've got no experience with impressioning levers I'd advise you to take it to a locksmith. Or you could buy one of the decoders lol... but for just one lock that'd be a bit overkill as they're about 400 quid