Welcome to DISboards.
Unfortunately there is no set rules to booking flights. Best price might be on release, 11 months in advance, other times its last minute. Could be anywhere in between. A survey earlier this year suggested 21 weeks before departure is the best time to book flights to the USA but there really is no set rule.
Going out of school holidays helps, also being flexible, look at all UK airports rather than just your nearest, look at more than just Orlando Int or Sanford, widen your search a few days either side of your planned dates. Being as flexible as poss will help find lower prices.