Section 55 British Transport Commission Act 1949: this penalises trespass on railway lines or property in dangerous proximity to such lines or electrical apparatus. Evidence is required of a notice exhibited at the station nearest the place of offence providing a clear public warning not to trespass on a railway. Punishable by a fine.
No signs - questionable whether they could enforce a fine or any other action. There's definitely a case to argue anyhow.
To be honest I doubt much will come of this. Chances are the people arrested will just get a smack on the wrist and that will be it. Lets face it they haven't damaged the railway or its infrastructure and there was no danger to the travelling public.
As for the response time, the BTP only have to come from Euston where they have a permanent presence, so 2 mins isn't that bad