Very effective, but slightly tedious way to find fantastic rates:
1) Check all the big pages - skyscanner.net, kayak.com, expedia, statravel etc. Make sure to use flexible dates and airports searches (eg 'nearby airports' features). Once you have found the cheapest flight on each route, note down price and all flight numbers and times.
2) Check which airline(s) that cheapest flight is with, and check their webpage directly. Sometimes there are exclusive deals on those pages not available elsewhere.
3) Check for connections to/from the nearest big hub - probably London in this case. Repeat steps one and two for flights to/from that hub, you may save a lot more than you'd pay for the internal flight or train to London.
4) Once you have found the best connection, give your local travel agent a ring and see if their price is the same.
Good luck!
PS: Also have a look for airline offers - often there are "book between dates x and y, fly between dates a and b" for big destinations like NY. If you are a little flexible or the dates match, these offers will be amongst the best you'll find. It's worth checking each big airline individually just for those
!
I try to take one day at a time, but sometimes several days attack me at once.