Scheduled Airlines will only start quoting fares from 10 or 12 months ahead and the better deals are sent by email newsletter which you can sign up for on the relevant airline sites.
You are trying to book up way too far ahead, Thomas Cook agents are just after their Brownie points for getting your booking and will tell you any reason to get you to book up now. In 11 US trips including this recession and the last one I have never booked up more than 4 months ahaead and once only 36 hours ahead. In June this year flights to Los Angeles longer distance) were been offered for £100 less than you were quoted for a shorter flight. I suggest you start looking about 6 months ahead and this will be plenty of time, if it were peak season like school holidays or Easter then an earlier booking would be advisable.
I sustained a bad injury to my leg after my last trip and had to book a First Class ticket to SE Asia only a week ahead of my schedule and Kayak got me a fare of £2840 which was £2100 cheaper than was being offered with other airlines.
The only time that airline fares get higher is if the airline sees the demand like Sunday flights (long haul) and Monday morning red-eye flights (short haul) as well as Easter, Christmas etc then only a small percentage of seats will sell at lower prices and then they increase the fare as more and more seats sell out. If you are a little flexible with your travel dates then you can even get better deals as some airline websites give different prices over a whole week.
Have just done a quick search for this year to compare, in late September United Airlines, booked through Ebookers are by far the cheapest @ £359 per adult inc all taxes and fees, and that is on a nice Boeing 767 wide body jet, some charter operators use aircraft with more seats and much less legroom.