It probably would have been a better idea to trade their USD market (if you were able to do so). From what I have seen, most exchanges do not have very liquid markets for fiat/BTC currency pairs except for USD.
Hopefully this did not cause you to loose large amounts of money.
Well, BTC/GBP seems quite small indeed (looking at coinmarketcap.com data). But BTC/USD is not the only high volume pair. Of course there is the CNY and EUR, but the Japanese yen and even the Indonesian Rupiah made some nice progress !
http://coinmarketcap.com/currencies/bitcoin/#marketsAnyone can trade any pair if they deposit btc and withdraw btc
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Anyway, this is an accident on coinbase, it should not be looking for any reasons to shirk responsibility. If you were trading on a exchange, which the market depth suddenly turned out to empty, and the price flash crash more than half, what a terrible disaster!
This is absolutely not a sensational, such a serious accident, it may occured tomorrow in other markets on coinbase, such as btcusd, btceur etc..
Stay away from coinbase (at least its exchange)