<p>In my personal experience at least 50% of US students studying in the UK completed a year at a US college first. In other words they applied while enrolled in a US college. UK unis sometimes prefer it as they perceive US high school as too easy. </p>
<p>You can always apply to start in the first year of any UK uni anyway. There is no real concept of transfer. You may start again with a clean slate at any time.</p>