Circling back on this after our trip to report that we succeeded in getting breakfast vouchers for a pre-park opening Leaky Cauldron Breakfast!!
We arrived at UOR on 30 June and checked into Portofino Bay, where I immediately headed to the guest services desk to ask about this. The TM confirmed that he had availability for the next morning. We bought the vouchers on the spot (printed out as four paper cards/size of a key card), and were told to be at the USF entrance anytime from 9am and look for the signs for the Leaky Cauldron breakfast. Regular park opening was at 10am. There was no specific time written on the voucher itself, so I bet it would have also worked later in the morning (though in my view that would defeat the entire purpose of getting it!)
The next morning we got in the water taxi at PBF around 8.40 and easily spotted some signs at the USF entrance (I forgot to take a picture). They signs had us loop all the way to the right of the taps, where we found a group of around 20-30 people starting to tap into the park shortly before 9am. Once everyone in the group had tapped in, we were told that a CM would walk us to Diagon Alley (and that we’d need to stay with the TM at all times, as the rest of the park was not yet open).
It was fun to walk through a totally empty park! Once we got to the London waterfront, the TM got us all to stop and huddle and told us that we’d walk into Diagon Alley together and then we should go into the restaurant to pick a table, scan the QR code and then place our breakfast order/hand over our vouchers by lining up at the counter. We were told that we were welcome to explore Diagon Alley one we’d finished breakfast but reminded that no attractions (or other parts of the park would open until 10am).
This was our first trip to UOR, and being the first guests to walk into an empty Diagon Alley was definitely a highlight of the trip for me. Most people just immediately filed into the Leaky Cauldron, but we lingered a bit and the TM took a family photo of us in front of the dragon. Breakfast was tasty and super relaxed, I would estimate that fewer than a quarter of the tables were taken. We had selected a table right next to the exit into Diagon Alley and popped out a few times to take more pictures of the empty land and get our first glimpse of the dragon breathing fire (wow!)
A second group of guests was led into Diagon Alley/the Leaky Cauldron by a TM around 8.40am (so I’m guessing they brought in the first arrivals at 9am and then anyone who arrived before 9.30 in a second round?) By this time we’d finished our breakfast and just started walking around the land soaking it all in and taking more pictures. A handful of other people were roaming like us, some doing spells (we didn’t have our own wands yet). Some of the shops appeared to just be opening, and at 8.55 or so a wave of people arrived from the regular rope drop (it seems the taps opened before 9am). We got in line at Ollivander’s for the first wand show of the day when we saw the wave of people streaming in, and were let into the shop at 10am on the dot (when the dragon breathed fire!)
While I don’t know if this experience is always available and how common it is to have short-notice availability, I would HIGHLY recommend trying to get these breakfast vouchers to any Harry Potter fans, especially anyone wanting to try out the Leaky Cauldron anyway. Getting there before park open felt very special, and guaranteed us an empty restaurant and an empty land to take amazing photos.
Note that the breakfast voucher includes drinks (including butterbeer and other themed HP drinks like pumpkin juice or fishy green ale). They don’t explicitly advertise this (the examples of free drinks listed are soda, milk, etc.) so just wanted to share as it makes breakfast a pretty good value (when a butterbeer on it’s own usually costs $9!)