Depending on the policy of the airlines, you may be able to get both boarding passes in England, and have your bag tagged all the way through.
You will still need to collect it in the US and take it through customs, but the process will be simplified.
I've done this quite a few times. I've often had one ticket (eg: from SYD-LAX) booked on Qantas, and another ticket (LAX-???) booked on American. Because they're both in the oneworld alliance, they will check you through even if it's not on the same ticket.
When I've done this my bag has been tagged to my final destination and I've received all the boarding passes in Sydney. When I get to LAX I simply clear immigration, collect my bag, leave the customs area and then drop my bag off again. As I already have my ongoing boarding passes it's just a matter of proceeding to the right terminal (usually T4 for me), clearing security and then relaxing in the lounge until my next flight.