Obviously you have to abide by 135 or what ever governing body you operate under.
I dont know why either of you quote "UK " rules, other than the OGP guidelines handbook hosted on a UK site.
I posted the link to the OGP because if you have never operated for an oil company in an offshore environment there is a lot of relevant information. Audit forms, crew qualifications, flight following , required training, equiptment, record keeping etc. Almost every oil company these days expects you to follow these guidelines.
Id say reading through this would be a good start if you want to fly for an oil and gas producer offshore.
RB