If it's of interest http://www.ppcoa.org...stabularies.pdf
There are independent port police forces for Tilbury (the remnants of the once mighty Port of London Police), Liverpool, Felixstowe, Teesport, Falmouth, Belfast, Larne, Portland (the newest), Dover and Bristol. Some of them have been around for a lot longer than the Home Office geographical police forces ;-)
Their jurisdictions did vary subtly, but generally it was a mile or two of the harbor authority boundaries (but that can be quite a big area) but there was a problem in so far as DP's have to be taken to a PACE designated police station for processing, which usually means driving some distance in a marked police vehicle with the DP in a cage and cuffs. Naturally if the police "powers" relied on evaporate a mile or two down the road and before the custody suite is reached there was a bit of an intellectual problem.
So it was recommended back in '08 that their jurisdiction be extended http://www.ppcoa.org...lice Forces.pdf either to match that of the Home Office forces (E&W) or to match the old jurisdiction of Home Office force's special constables, that is geographical Force and contiguous Force areas.
A Maritime Security Bill was drafted, but a load of things got de-scoped including that proposal. A private members Bill was then advanced, and pleasingly (As I made the recommendation in the first place!) it resulted in their jurisdiction being extended to England & Wales on 26 June 2013. http://www.legislati...013/23/contents and http://www.legislati...9/contents/made