It takes about eight hours to fly from the USA to the UK, which depends on departure and arrival destinations. For instance, to arrive in London, you will need to spend about nine hours in the sky. Also, you should remember that you will have to spend some time at airports.
The price for a flight from the USA to London is about $660 for an economy class, but it also depends on where you travel from. For instance, a flight from NY John F Kennedy International to London Heathrow will cost you about $300. It seems to be affordable, but you can surely expect to be totally exhausted after such a flight even if you are happy about finding the cheapest flight.
Spending many hours in a small uncomfortable seat will not be pleasant, and it is not the only inconvenience waiting for you in coach class. Therefore, the other options you should consider are business class to England and first class to England.