It is not possible to have a gay marriage ceremony on a beach in  England and Wales as licensed locations have to be part of  building, not in the open air or under a marquee.  This is according to the law as defined in the  The Marriages (Approved Premises)Regulations 1995.   Of course, gay weddings are currently not allowed in Scotland or Northern Ireland, so that rules them out.

You therefore have two choices.  Firstly, you could have a minimalist legal ceremony at a licensed location then have your own ceremony on the beach with all your friends.  Secondly, you could go to a country that allows gay weddings and lets you get married on the beach.  The USA is a popular location for beach weddings, but the regulations do vary by state.