I opted to purchase in advance at the online discounted price and had them mailed to me. They arrived in about 5 days, I'm in PA.
With tickets in hand I hope to avoid at least 1 line when we get there!
Of course if you are already traveling, this won't help you!
This is what it says on the website:
You can choose to print your tickets at home (‘Print at Home’), collect your tickets on the day of your visit or have them posted to you. We have insufficient time to post them to you if your visit is within 7 working days or 10 working days (non-UK).
If you choose 'Print at Home' (currently Tower of London, Hampton Court Palace and Kensington Palace general admission only), please print out all tickets individually and show at the point of entry, failure to do so will result in delayed entry. Please note we do not operate a fast track entry system, you will need to join the queue to enter the palace.
At present we do not accept tickets on mobile devices.
If you choose to ‘Collect on site’, the credit/debit card you pay with and confirmation email/voucher will need to be shown when collecting tickets from the Ticket Office before going to the gate.