999 if you're attacked at a UK station and need emergency assistance. Anywhere in Europe (including the UK), 112 will also work for this purpose.
If you're not in immediate danger and don't need emergency assistance, you may also be able to call the new non-emergency number, 101. I don't know if that's been rolled out across the whole country yet but it's for reporting crimes to the local police. That would include theft and so on. Whether you would be put through to the BTP or not, I don't know.
Also, some stations have emergency help points. Selected National Rail stations have them, but equally so they seem to be at many non-NR stations or stops on other networks including the Tube, DLR, Tramlink, Midland Metro, NET, Manchester Metrolink and Tyne & Wear Metro.