If you kept it in a Faraday cage, probably nothing, but otherwise:
1. GPS - Just the equivalent of A-GPS would work - sniff the relevant
frequencies out of the air and send them to a back end server to work
out the location. If you're in clear enough range to do this then it
would probably also be useful for the network to use for local cell
planning and strength adjustment.
2. Look for other mobile networks - if you see non-British networks
(edge cases around Dover aside of course) then it's probably not in the UK.
3. You might be able to still do something with IP addresses -
particularly if you start using UDP or some such which can't be so
easily tunnelled. Or even start looking at round trip times which are
typically hampered by a tunnel.