If you mean the volcano, it sounds like Tom Baker to me ... actually the entire advert bugs me, it gets my vote for the worst advert currently on tv. Total rubbish!
I recognise both voices tbh but I don't think it's Tom. I thought maybe Brian Blessed (would he stoop so low??) or the guy who plays Dixon Bainbridge on the Mighty Boosh.
Hi there! I'm not sure who plays that character that you mention from the Mighty Boosh but I'm pretty sure that the voice is that of the actor Alexander Armstrong. I think he originally started on TV as a comedy performer/writer but more recently he's done some drama, see more here http://www.imdb.com/name/nm0035602/
I'm sure he's the voice of the Volcano but not sure about T-Rex Alan? And PS I have to say, I LOVE this ad!