You guess the GMP were warned first. Case solved!
BTP is responsible for railway policing in England, Wales and Scotland. Unlike Home Office police forces, the majority of BTP activities take place on private property, albeit usually private property to which the public is allowed access.
S. 31 of the Railways and Transport Safety Act 2003 (as amended) provides BTP constables with the powers and privileges of a Home Office constable within the railway jurisdiction (i.e. over all railway property and outside railway property when related to railway matters).
Upon said property BTP has responsibility for counter-terrorism (firearms officers were introduced in 2012), investigations of major crime such as rape and murder, and the many other duties undertaken by territorial police forces such as GMP which can render assistance upon request. The GMP has no power to police in BTP jurisdiction upon a whim.
http://careers.btp.police.uk/about_btp/specialist_teams.aspx