I use coinbase, they are more of a bitcoin wallet than a broker but they recently opened up gdax.com which has a trading platform and is a sort of exchange. I often pay a premium to buy bitcoin through my debit card (3%) as transactions appear instantly. Transaction fees through gdax is 0.3%. Anything above your daily limit and you'll need to transfer funds from your bank which takes me 3 days. Kind of annoying as the price of bitcoin can be quite volatile.
This all being said there are CFD and spreadbet brokers that allow you to trade bitcoin.
