The busiest London termini can be genuinely difficult to navigate for a first-time visitor, particularly at rush hour or during a major event nearby, when a station concourse can be packed with commuters and few obvious landmarks to meet at. Agreeing a specific, well-known meeting point in advance — a particular entrance, a taxi rank, a landmark inside the station — rather than a vague "outside the station" removes most of the risk of missing each other.
For an arrival by train connecting on to a flight, it is worth building a little extra time into the connection beyond the train's scheduled arrival, since delays on the rail network are outside anyone's control and a tight connection at a busy station adds unnecessary stress. The same logic applies in reverse: if you are catching a train after a transfer, allowing time to find the right platform at a large station like Paddington or Waterloo is sensible, particularly with luggage.
Stations connecting directly to an airport — Paddington to Heathrow, for instance — are also worth mentioning at booking if your journey combines rail and air, so the whole trip is planned as one connected journey rather than two separate, disconnected legs.