In fact, you want p2p content to come in from
the 'net because it only transits BT's wires once...I can't think of an obvious way for a p2p client to detect this.
What, you want to guarantee it's not coming from the local net? Easy,
prefer IP addresses that aren't allocated to the same RIR that your own IP
is.
Network engineers at global carriers can now go ripping their hair out
in frustration at the idea of all P2P traffic moving to a non-localized
model 
... JG