I trade with IG (ig.com). They have BTC/USD, ETH/USD and BCH/USD pairs. For BTC 7.5% margin. Spread is a little bit high at 20$, but with the high volatility in BTC it's not that bad.
Another broker I know is Plus500, they have even more crypto pairs (BTC, BCH, XRP, XMR, LTC, ETH, DSH, XEM). For BTC 3.33% margin. 18$ spread. I have never traded with them though.
Interesting.
Positive Points
+FTSE 250 company with a market capitalisation of £2.1 billion (31 May 2017)
+Regulated by the Australian Securities and Investment Commission (ASIC)
+Offices in 14 countries across five continents
+185,800 clients worldwide (May 2017)
+Average of over five million transactions globally per month (Apr-Jun 2015)
+Net global trading revenue of £491.1 million1
+Over 1500 staff around the world (May 2017)
Negative Points
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"IG.com is operated by IG Markets Limited. ABN 84 099 019 851, AFSL 220440. Derivatives issuer licence in New Zealand, FSP No. 1892
-The information on this site is not directed at residents of the United States or any particular country outside Australia or New Zealand and is not intended for distribution to, or use by, any person in any country or jurisdiction where such distribution or use would be contrary to local law or regulation."