Most sports have a residential qualification, as well as (or instead of) a birthplace qualification. With many sports being played at the highest level in only a fairly small number of countries, it makes sense to do so. For example, a brilliant rugby player might have been born and raised in (say) Iceland. It would be unfair to deny him the chance to play at international level just because Iceland doesn't compete alongside teams like England and France.
Cricket has always accepted overseas players into the England side after they've met the residential qualification. The England team for the 1992 World Cup Final included Allan Lamb (South Africa), Graham Hick (South Africa), Derek Pringle (Kenya), Dermot Reeve (Hong Kong), Chris Lewis (Guyana) and Phil DeFreitas (Dominica). Squad members who weren't selected for the Final included Gladstone Small (Barbados) and Robin Smith (South Africa).
The present England cricket squad is captained by Andrew Strauss (South Africa) and includes Matt Prior (South Africa), Kevin Pietersen (South Africa), Craig Kieswetter (South Africa) and Eoin Morgan (Ireland).
There would seem to be no reason why rugby should be any different. The All Blacks regularly feature Fijians, Tongans and Samoans in their teams. As well as Manusamoa Tuilagi, the England squad includes Dylan Hartley (New Zealand), Simon Shaw (Kenya), Riki Flutely (New Zealand), Hendre Fourie (South Africa) and Matt Stevens (South Africa).
Chris