Unlike many other places, the US does not provide any facilities for airside transit so you need to have the right to enter the country in any case.
Technically you don't necessarily need a visa for that but you do need something. As a UK citizen, you can probably apply for an electronic authorization (apply through the ESTA) and benefit from the “Visa Waiver Program” (or “VWP”), which is much less onerous than an actual visa but does also need to be taken care of in advance. The US state department's website provides some info on eligibility for the VWP.
Since you have to go through customs/border check anyway, you can indeed leave the airport or do a little sightseeing during your layover without any extra paperwork.