UK sunscreen will be fine to use in the US, and it is less expensive in the UK, but remember that you will need to use quite a bit for adequate protection in the Florida sun. For a family of 3, you should expect to use at least 180 ml. each day. (That measure would not be different for sunscreen purchased in the US; the amount is the same regardlesss of what type it is. 180 ml is about 6 oz. Most sunscreen in the US is sold in 6 oz. bottles; that is a day's worth for two avg. adults and a small child.) You'll need to reapply every 1.5-2 hours.
For shopping comparison, my favorite US brand, Coppertone Sport Sunblock Spray, costs about $7 for 6 oz. at my local
WalMart or Target, but will cost about $12 in the chemist shops near WDW. (In Florida, you will always pay an inflated price for sunscreen in shops near beaches and theme parks. Regular Florida stores will be cheaper.)
BTW, For faces, I recommend using a solid sunblock stick. Liquids tend to run into one's eyes when it is really hot, and that is quite painful. A bit more expensive to get a separate stick product for faces, but worth it, I feel.