If applying through the US, use the American resume style. If applying directly through the UK offices, use the UK CV format. I find the CV gives a bit more room to display your qualifications, attributes, and any other information that may be of interest (hobbies, official publications, etc). Also, HR may not "obviously" be English, but you would want the style you choose to match that of other applicants.
I recently accepted a job at the UK subsidiary of an American company. I've been in contact with 3 HR staff-- one in the US, one in the EU, and the third in the UK.