I'm a British citizen travelling from London Heathrow (LHR) to the Hartford, Connecticut airport, and have a two hour stopover in Newark, New Jersey (EWR) on my itinerary. At what stage would I clear immigration and customs on arriving in the USA?
Generally, you clear US customs and immigration at your first point of entry into the country. In your case, that is Newark.
There are a few airports outside the US that offer preclearance flights. A preclearance flight is screened for US customs and immigration at departure, rather than arrival, so you arrive in the U.S. as if on a domestic flight. Preclearance is only available from selected airports in Canada, Ireland, the Caribbean, and Abu Dhabi. London Heathrow is not among them.