A lot of the other posters have suggested that he's using the Received Pronunciation, but that's not entirely correct.

Oftentimes we confuse the standard English of the Southeast of England as being identical to RP, and although they've many similarities since RP is based upon the English of the Southeast, it's also quite different in a number of ways. The most notable difference is in regards to the pronunciation of vowels — when speaking in RP, one would pronounce the words mouse and house as /maɪs/ and /haɪs/ (rhyming with rice) as opposed to /maʊs/ and /haʊs/ as someone speaking in a Southeastern Accent that isn't RP. In addition to that, RP has a very distinct pattern of inflection, strong nasal tones and a tendency to pronounce foreign loanwords into English using their native pronunciation as opposed to the Anglicised usage, for instance ballet would be pronounced as /ba.lɛ/ and not /bæl.eɪ/ which is the norm for most speakers of English from England.

I've not seen Ewan McGregor portray Obi-Wan Kenobi, but I've listened to some soundbites from the film and I think that the most accurate answer would be to call it Southeast English, it exists as a midpoint between Received Pronunciation and Estuary English.