Spies arrange meetings all the time. Just agree on the place.

How about the under the left side (one reads left to right) of the destinations and platform number board which is not too far from platform 13 and 14.

Make sure you have a password to deter imposters.

"The birds fly south this time of year"
Answer: "But they come back in the spring, never fails."

If they forget "never fails", they are imposters. Shoot to kill. You have such a license 008.

See http://www.vrlondon.com/vr/london/stns2.shtml