Hi Jill,
How fantastic that you will be celebrating various family birthdays as well as your 40th birthday at Walt Disney World!
It is absolutely fine to celebrate your birthday during your next visit even if it has been your actual birthday before your trip or won't be your actual birthday until later that year - especially as you will be coming all the way from here in the UK!
Be sure to pick up some birthday buttons (or badges as we would say) when you check-in if you are staying at a Disney resort hotel or alternately from Guest Services as you enter one of the parks.
To make your visit extra special, perhaps you could consider making an advanced dining reservation at a signature restaurant for a special birthday meal and maybe even order a celebration cake to mark the occasion by calling 001 (407) 827-2253 at least 48 hours in advance.
Another idea would be to arrange for an In-Room celebration or gift to be waiting for your family upon your arrival if you are staying at a Disney Resort hotel through Disney Floral & Gifts. (www.disneyflorist.com)
You will also find lots of other ideas on the
Customize Your Special Occasion page of the Walt Disney World website.
I hope that this has given you some ideas and wish you a magical holiday and lovely birthday!
.