Given that joel campbell could play in greece and spain but at the time was not allowed to play in the UK, then it seems to me that uk takes a much tougher stance with workpermits for players from non eu countries than other countries in the EU.
It's the UKs» own workpermit laws which dictate the criteria for issuing these, that's why we have to send the likes of Joel Campbell out on loan to a more lenient EU country for two years to enable him to qualify for a UK workpermit.