Max current draw from a UK Phone Line?
This source states that BT provides 40mA when the phone is off the hook.

This other source talks about the expectations of the ringer circuit:


less than or equal to 5mA draw at 35V/25Hz ring signal
less than or equal to 10.5mA draw at 75V/25Hz ring signal


PowerDsine's North America ring module generator appears to be limited to 3W, so I would expect the PSTN provider's capabilities to be similar. This signal gets shorted out when the phone goes on-hook, so the source can't be too stiff.