Why does San Jose have to be moved to Heathrow ? It's likely to be aimed at Europeans who want to go on holiday to Costa Rica to see a bit of jungle / wildlife and then spend some time on the beach. San Jose as a city really isn't that great a place to visit.
Costa Rica has not traditionally been a place where the very wealthy crowd go on holiday (e.g. Barbados or Indian Ocean islands) and I doubt there's a huge amount of business travel between Costa Rica and non-UK Europe which will be better served via London compared to Madrid. Seems to me like a good solid candidate for a long haul leisure route at Gatwick for years to come...