As an American, I find the following traits give away British families at Disney immediately:
1. Men have the English soccer hooligan shaved heads (think Jason Statham). I find it odd they would get their hair cut so short just before coming to Florida where the sun is very intense. That extra hair would otherwise provide great protection from a terrible sunburn on the scalp.
2. Women have what I call the "British bob" hairstyle. They're trying to emulate Victoria Beckham, which means their hair has to be long on each side, but very short in the back. It's also called the Posh Pageboy. Click on the link and scroll down to see what I mean.
http://lifestyle.ezinemark.com/10-best-hairstyles-of-all-time-77364b9c37f9.html
3. Children are wearing Manchester United shirts.
4. All family members have footwear that isn't sold in the US--mainly athletic shoes made by European companies.
5. When a family descends from a car in a Disney parking lot and immediately creates a cloud of spray-on sunscreen, they're British.
6. Walking "against the flow" usually means the person or family is British. We walk as we drive--on the right-hand side.