If you are sure that the your greek friend was driving, then you must name him on the S172. To do otherwise would be an attempt to pervert the course of justice.
However, so many have tried blaming fictitious foreign nationals that the scammers will probably smell a rat. One issue is that, because you are not sure that your friend's address is valid, you might be seen to have loaned your car to a foreign national who needs his own insurance without checking his insurance document and thus failed to exercise due dilligence to establish the identity of the driver. There could be a perfectly good reason why his address doesn't seem to pan out - but the fact it doesn't could leave your position compromised WRT an S172 charge but not to one of speeding.
Your best action is not to fill in the S172 right now. Get over to
PePiPoo, fill out the NIP wizard, and post the results when you ask there for advice.
Good luck,