Lisa,
Just realize you'll be visiting in July which is very, very hot, humid, and rainy in central Florida. You'll need to go at a slower pace during the heat of the day, and even early morning and later in the evenings can be rough on the heat and humidity. You won't be used to it being from London, so just take care that you don't overdo.
Now, as for your schedule, don't "set in stone" what days you visit what parks until you look at the calendar for the days you'll be there. You didn't mention specific dates you'll be visiting, but if you are not staying at a Disney resort during your visit, you'll want to avoid the parks having Extra Magic Hours for the Disney resort folks, as these parks will be more crowded due to that fact on those specific days. Also, you want to avoid Magic Kingdom on Mondays if possible.
Here's a link to the calendar for the parks; check each park to see what days EMH is and is not occuring for the Disney guests.
http://www.wdwmagic.com/calendar.htm
I would not attempt to do Epcot and a water park on the same day either. Epcot is the largest park in foot-traffic territory and you need a full day there to give it justice to see both Future World and the World Showcase area.
If you simply must combine a day at a water park with a day at a theme park, do so on either your Hollywood Studios day or your Animal Kingdom day instead. Personally, because of the threat of bad/stormy weather every single day in July, I wouldn't bother with a water park unless you can go to it first thing in the morning and leave before the storms in the afternoon begin.
Yes, during July, fireworks are done every night at Magic Kingdom...and Epcot...and the Fantasmic show at Disney Hollywood Studios, and all these shows are worth the effort to see. Why not just stay in the parks and see them rather than leaving to go elsewhere in the evening hours. Then, if the rain sets in and it appears they won't be done, then you can go to DTD or elsewhere.
Do you have your park tickets already bought yet? If so, you need to set up a My Disney Experience account on the WDW website and scheudule your Fast Pass+ selections for the days you'll be visiting each park. Now, if you don't yet know what days you'll go to what parks based on the calendar I gave above, it might be wise to begin planning. July is very, very crowded and FP+ is a necessity for many of the major attractions if you don't want a 2-hour wait or more.
Hope this info will help you a little. Visiting Disney World isn't a trip you can just "wing it" and still get it all done in only four days. You really need a plan and an efficient one at that.