Last year we got a late booking for flights from Manchester plus villa (package with Direct Holidays) - £1300 for me, DP and DS age 6.
Booked car through US rentacar and got an SUV for 2 weeks for @£225. Do not take the car that the tour operator offers - it is NOT free; you pay way over the odds for insurance.
Other major cost is park tickets. Buy them in UK - much cheaper. Loads of websites. Think we spent @£550, maybe more but we did all Disney parks and Universal Studios.
One thing that made a difference was putting DS as an adult for some strange reason. It knocked @£300 off the package price. Had to phone to book it this way though. We ended up with a lovely 5 bedroom/3 bathroom villa overlooking a golf course.
We flew late August and DS missed first 4 days of school in September, not ideal I know. However parks are much quieter as US schools go back mid August. If you are in England and can get to Glasgow keep an eye on the prices as Scottish schools go back mid August too.
Sorry, see you want to go in May. Have a look at flying via New York as that sometimes works out cheaper. Or even consider flying into Tampa although that is a considerable drive to Orlando on top of a long flight. Flight prices can change daily so it requires lots of research.
If you do book, watch the £/$ exchange rate as it is very favourable for Brits at the moment so you might want to buy some currency while the rates are good.
Finally, if you do find a bargain please let me know.