Just updating with my experience and rationale for doing this in case it helps someone else - firstly Thank You so much to ThBa and everyone else for contributing to this thread - fantastic information
We are a family of 5 however only 4 of us going on our next trip - using the postal service means we benefited from the family discount of 20% which we would not be able to do in the park as DS isn't coming with us. We are also shareholders so get an extra 15% on top of this. So 5 Classic passes cost in total £397.08 at current exchange rate.
Classic passes are fine for us as we always stay on-site so will still get EMH. As shareholders we also get a higher discount on restaurants/purchases than the classic pass.
I filled out the form with mine and DH info. I put the same info for our 3 kids on a separate sheet of A4 paper. I used passport photos for the kids but didn't have any to hand for me or DH. So I used a normal photograph and just cut it to size of the passport size. I wrote our names on the back of the photos and then glued them to the form and separate sheet of paper.
I enclosed photocopies of our UK passports and a copy of proof of address for me, DH and my eldest kids (both over 18). I sent copies of letters about college acceptance with the kids names on and a copy of a household bill that had both mine and DH name on it. I didn't send any proof of address for youngest DS (12). I also enclosed a copy of my shareholders card.
On the form where the amount goes I input the cost of the 5 passes less 15% discount and wrote 'shareholder discount 15%' beside it.
I filled out the payment part of the form with my UK debit card details.
Posted all using registered/tracked post on Thursday 1 September.
Tracking showed received on Monday 5 September.
Bank was debited with amount on Friday 9 September.
Cards arrived today, Thursday 15 September - exactly two weeks after posting.
I didn't receive any communication/email until they arrived today so I'm glad I sent it tracked post otherwise I'd have been wondering if it arrived safely.
The cards are dated 9 September, so must get dated on the day the payment is taken.
Thank you again to everyone for being so helpful. Initially this sounds difficult to do, but actually it's very easy to do. Can't wait to get there now and flash my card