Sean,
First something that is kind of a technicality, but significant IMO, is that RCPGC does not have "country" memberships. All members are full members of the club and have full playing rights. For many distance members -- OS or merely elsewhere in Britain -- the autumn and spring meetings (held on bank holiday weekends) are the prime time to visit the club. Lots of old mates there to catch up with and an honour board event to play for.
But to the issue you raise above, what you may not realise is that by virtue of its "distance from club" categories, Londoners are effectively in the "country" relative to Deal. Coupled with the easy 1h45 motorway drive or high-speed train trip from London, the quality of the course and club and Deal town itself, it's easy to see what many London golfers choose Deal as their second club.