Weekend flights are usually more expensive, booking direct with BA or virgin always seems to be more expensive that using a third party to book the exact same flights in my experience. I use several comparison sites for flights and get alerts sent when the prices change. Four of us flew direct last December for £1270, then again in May this year for around £1500. I am currently looking at 16th June for just over two weeks, the lowest I have seen Virgin from London direct was just over £1400 for all four of us. My boys are 13 and 14 so you do save a bit on the Tax for their flights. Admittedly we fly a week or two before the end of term most of the time which has a huge impact on the prices.
I read somewhere that the best time to get the best deal is around 4 months before the date you want to fly.
Always use a flight comparison site for the best deal, I have booked via several different agents Travel up, Netflights etc, and always less than what the flight company is selling for on its website. Beware Norwegian charge for everything, baggage, food etc on top so not always as good as it looks.
Another thought, we have flown indirect before and this makes the immigration thing much easier, you do the immigration at the airport you first land in, get on another flight within an hour or two and once you land at Orlando International airport you are own your way. One time staying at Animal Kingdom Lodge we even checked our baggage in at the hotel in the morning, went off for the day and didn't see the cases again until we got to London. You need to fly with certain airlines to do this, but it was super easy both ways. The lines at Orlando international airport on there last two trips were terrible, it took hours, I worked out the travelling took as long in the end as it would have done if we had flown indirect, purely because of Orlando Airport. Some of the American airlines I have found to be very good, no hidden charges.