Start a Stripe.com clone for the United Kingdom. Even better, launch to the wider EU and support accepting payments in GBP, USD and the Euro. You’d instantly get worldwide press and massive attention from developers and startups within the countries you’ve chosen to support.
Stripe is too busy focusing on the USA despite saying they’re coming to the USA and other competitors in other markets are starting to spring up, like the newly announced PIN which caters to Australians.
Your competition will be incumbents like SagePay, Paypal, and Google Checkout, all of which won’t be able to move fast enough and are despised by your target market: startups and developers. The only real threat to you in the United Kingdom is GoCardless which could attempt to compete, but they’re focused on UK Direct Debit payments at the moment.
The lack of competitors doesn’t really matter anyway – there’s plenty of room for you, plus the above, plus Stripe. My training company software startup would use you in a heartbeat. Dozens more would as well, just troll the forums begging Stripe to launch internationally and scoop up your first beta testers. Seems all you need is a couple smart developers, a good lawyer, and a connection to a bank.