Voice accents are not based on the place of boarding or landing.
That would be wrong, of course, since it would result in unrealistic situations like a United Airlines flight starting from, let's say, Dubai, turning to English/Arab accent for that flight only.
Crew accents (we don't have passengers voices anymore, only Crew greetings) are more properly related to the nationality of the airline, so Virgin Atlantic being a British airline, will always have a British accent no matter where they fly from or to.