What is your leverage set to, Simon? You would have had insufficient funds for the trade you tried to enter, if you were trading with insufficient leverage? (That little “order entry window” says - on the top line of what’s printed at the bottom - the number of units available for you to trade, and it’s less than the number of units you were trying to trade). I’m guessing your leverage is set to 10/1, and you might want to change that to 50/1. The stop-loss you enter yourself isn’t the one that Oanda uses, for that calculation:[I] for purposes of permitted position-sizing[/I] they use a much, much wider stop, just to allow for “sudden unexpected disasters/news” causing a fast-moving market.
Also - though nothing to do with your question above - at what time of day were you entering a trade on Cable? 
Edited to add: also - it’s a Bank Holiday in America, today, so I’m not trading Cable, myself: [B]301 Moved Permanently