Hi Emma,
It's lovely to see a question from a fellow Brit and firstly, let me tell you that you are going to have an amazing time at Walt Disney World!
Questions about spending money are always difficult to answer as everyone will have different budgets and opinions.
As you already have your park tickets and Disney Dining Plan, you will only really need to bring enough money to pay for gratuities at the Table-Service restaurants and to buy whatever souvenirs you would like to take home with you.
Typical gratuities at all Table-Service restaurants at Walt Disney World (including Signature Dining) are 18%, assuming that you have received Disney's normal high quality of service. I would recommend that you take a look at the menus for some of the restaurants which you will be dining at during your stay on the unofficial website www.allears.net. This should enable you to make a fairly accurate estimate as to what your gratuities will cost.
To help you with your planning, I would recommend that you visit the
My Holiday Planner website where you can watch a number of videos including my top tips which are aimed specifically at helping UK Guests plan their trips to Walt Disney World.
You will also be able to order a copy of the Little Big Book of Magic guidebook which contains pull out maps and insider tips to help you get the most from your trip as well as a Walt Disney Travel Company brochure and some other planning tools.
Wishing you a magical holiday filled with memories which will last a lifetime!