Virgin Mobile certainly has the best customer service I've experienced. I've been with Fresh (from Carphone Warehouse) and Orange in the past, and after not being able to get through to customer service from either company I was pleasantly surprised to get straight through to Virgin Mobile
They thoroughly deserve the best customer service awards they've won for several years in a row! As far as I know they are still based in separate call centres from the ex-ntl:Telewest services.