Hi There, I work in the audiobook industry and use lots of voice over artists, yes it's true that a big name will sell more books and is more likely to get better work, but people like educational publishers making English Language Teaching Tapes use a lot of unknown actors. I'd recommend that you try and find yourself an agent and send them a show reel, it doesn't have to be posh, just a few minutes of a straight read, then examples of your accents will be enough. If the agent thinks you have talent, they will arrange for a more professional recording to be done, as often now voice clips need to be present on the agents website. My favourite London Agents are Voicesquad, Hobsons, Rossmore, Calypso. Vincent Shaw associates take on a lot of less experienced people. You should also volunteer to record an audio book for Calibre Audio Library, you won't be paid, but it will give you experience and you can see if you enjoy the work.
Good Luck!!