Christmas flights?

We are planning on going back to the UK in December for 3 weeks over Christmas and New Years. We do not want to buy too far ahead or too late and miss out as there will be 2 adults and 2 kids, so we need to make sure we are sitting together. Now the prices are about $650 and wondering when you bought and how much you paid. Also any sites that have good deals. Thanks a bunch!

where in the US will you be flying from?

Indianapolis

You could try airlinesnetworks.co.uk (airlines-network.co.uk)   www.cheaptickets.com www.expedia.com www.orbitz.com
www.hotelclubs.com www.opodo.co.uk  For rental cars in the US I would recommend www.avis.com www.hertz.com these two are very accomodating.
I have about 40 websites I can link you up to. I am what most people would call a professional traveller. For not having a job that puts me on a plane all the time...I have flown about 35 times in the past two years...half of which have been around the world.
My fiance bought a ticket to america on November 1st of last year and flew out 16 deac-8 Jan. If you want to limit your customs experience I recommend a direct flight.
if you do have a layover...please ensure it is for at least 2-3 hours. Some airports like chicago and Detroit are horrific to travel through. atlanta is nice anytime of the year as is denver.

My boyfriend and I didn't buy our tickets until November last year. We bought tickets for about $800 round trip direct from Boston to Heathrow.

We bought ours in Febuary (early for us) - direct to Seattle on BA (always expensive anyway) for £400 each.  That's flying in the 17th December and out the 1st of January.  It's gone up to £460 now.