Well, it seems to be listed in their Listed of General Procedures on their website - [broken link removed]. It's on page 34 (numbered as Page 33). If you do a search in the document for Breast Reduction, this is what comes up.
It says it requires pre-certification unless it's for reconstruction following a mastectomy, so your doctor/consultant will probably have to send them something before you know for definite whether it's covered or not.
It's probably no harm calling VIVAS anyway, because they can check if waiting periods are served/applicable, what kind of hopsital cover you're entitlted to, is your consultant recognised, etc.
In any case, best of luck with the procecdure!