Not sure which rules and regulations and codes of practice (mmm… that’s in bold type. Wonder if it’s an important term I should remember/use?) might apply to your poster(s)? No worries… You’re maybe looking in the wrong place for help.
The main legal constraint (another good phrase that, you should maybe use it at some point! Hint. Hint.) from an institutional point of view (Another great phrase – where does he get them all!) comes from the ASA. They are the regulatory body who rule on the suitability of an advert (which is, after all, what a poster is – an advert for a film).
When you are designing a poster, there is regulatory guidance available from the CAP (https://www.cap.org.uk/Advice-Training-on-the-rules.aspx). The CAP are the Committee of Advertising Practice and they offer training and guidance on advertising… and they also draw up the rules/guidance that advertisers need to follow. They work hand-in-hand with the ASA.
Now, I had hoped that at least one of you would have stumbled on the CAP by now… and specifically, this page: https://www.cap.org.uk/Advice-Training-on-the-rules/Help-Notes.aspx
But just in case you haven’t, and want the tl;dr version, you NEED to look at this guide in particular: https://www.cap.org.uk/Advice-Training-on-the-rules/Help-Notes/Videogames-Guidance-on-ads-for-videogames-and-films.aspx#.WLSGTBicbrc