If you live and work and were brought up in the UK, start with the UK. Here's why..
I work in both the US and the UK. The entire language of selling is different in each country, as are product descriptions. A UK "Remortgage" is a US "Home Refinancing Loan", for example.
By working in the UK, you stay with familiar selling messages and understand both your buyer and the product. This means that you can pull the right emotional strings with your marketing messages, be they ads, onsite content or the keyword terms you rank for.
I've only succeeded in the US after using direct product and copy advice from merchants, whose very different approach to selling was a surprise to me.
So stick with the home market until you have a steady income stream, then use your newly learned skills to address the US market later, and from a position of more experience and financial stability.
That's what I'd do anyway :)