Hi Stephen!
Welcome to planDisney.
It's always a pleasure to assist my fellow guests from the UK, and I am very happy to talk about this because you have a few options.
If you have purchased your
UK Magic Tickets (7 or 14 days) as part of a package with a hotel reservation, the start date of those tickets will be your hotel check-in date. If you buy the tickets as a stand-alone purchase, you can select your start date - but I realise this is not ideal because you wouldn't be eligible for the
Free Dining & Drinks offer for 2025 if you have separate hotel and tickets purchases.
What you could do is make a 'hotel only' reservation for the first 3 days, when you'll have no tickets and no free dining. Then make a hotel & ticket reservation (thus qualifying for a free Disney dining plan) for days 4-21 of your trip. You can spread your 14 days of park admission over those last 18 days, and you can enjoy park admission up until your last day.
If you want to add park tickets to the first 3 days, you can always buy them locally on the gate, at any Guest Relations window at all the Walt Disney World theme parks, water parks, and at the Ticket Center at Disney Springs. You can also buy tickets online in the US at https://disneyworld.disney.go.com/admission/tickets/theme-parks/.
Lightning Lane selections can be purchased without
park reservations when using your 14-day Magic Ticket. If you decide to purchase any tickets for your first 3 days (as I suggested above) park reservations are not required for date-based tickets. You will be able to make your Lightning Lane purchases once you arrive in the US.
Please come back and visit us if you have any further questions, and you can
send me a direct question if you wish. We are always happy to help you plan your magical holiday to Walt Disney World!
Sue