The simple answer is that they can ask you to leave and use 'reasonable force to do so'. If you persist they can call the police to help remove you. MOD land and Railway land is different. Itis possible to take an injunction against someone to stop them trespassing; if they break the injunction they could be jailed for contempt of court.
From memory, you are fishing tidal waters (river Hamble?). The foreshore between high and low water is crown property generally and there is access provided you can get to it in the first place!All this may change with new laws that are aiming to increase access to the foreshore on tidal water though there are many aspects to consider like SSSIs, estuaries, marinas etc.
Be a bit more specific on what you are being prevented from accessing and it may be easier to help you.