if the volumes do not suit, there are many larger exchanges. For example, they do not accept a deposit in pounds.
I don’t know if I’m getting you right, but do you mean that you’re looking for an exchanger where you can buy at the best rate? I think you should try out local Bitcoin, cause I believe there is an exchanger for every location on their site, whether UK, US, Asia, Africa, or Australia. Though I’ve never done business with any exchanger on local Bitcoin, I prefer to do it someone I know face to face. If you would use local bitcoin, sell and buy from exchangers with good ratings.