afaik tomorrow and the day after (22nd + 23rd July) are the last days of this deal, so if it appeals to you, get to it soon. Also idk if every branch/chain/whatever is participating.
I won't repeat much of the basic terms and conditions, but you do need to make an account on their site and generate a QR code before you start. One QR code is enough for a group, although obviously each person in the group will pay their own separate fee.
It's £12.50, you get 90 mins from your first order, that 90 mins includes cooking time and time to get served which sounds like a scam but in practice it's probably not going to be your bottleneck unless you're a professional eater. afaik each person is only allowed one QR code but the phrasing suggested nothing stopping e.g. two people going two nights a row with person A supplying a QR code on night one then person B supplying a QR code on night two. Sign and site says 6pm-9pm, but they let me make my first order at 17:45, no idea how strict they are with the 9pm ending, and it depends on the mood of the staff regardless ofc. SORRY, correction, you must START before 6pm, idk how I fucked that up, genuinely didn't realise until someone corrected me just now in the comments. Makes a lot more sense in hindsight, they already get decent business after 6:30pm and want to fill the empty store before 6pm.
Overall you won't have any issue having enough time to order and eat ~3 pizzas unless you're a very slow eater or have a small stomach, which at that point it's clearly a good deal imo, even two pizzas wouldn't be terrible by any means.
However, they ran out of vegan cheese so I "only" got 4 pizzas, so ask specifically if they have enough for e.g. 5 pizzas, before starting, if you really think you can eat that many (or going with multiple vegans).
On that topic: 4 pizzas was more than enough, if I had a 5th, which I nearly did, even cheeseless, I'd very unlikely enjoy it. The salt alone was killing me, figuratively and (to a degree) literally.
If my stomach was bottomless and they hadn't run out of vegan cheese then maybe could have had 6 pizzas, maybe some of the last one would go in a takeaway box (which is allowed "if not abused" whatever that means).
So yeah, for me, a very large stomached person, the bottleneck was easily my stomach, I left 20 mins before my time was up. I was patient with ordering, you're allowed to order a new pizza when your current one is over half way eaten, but all but one time I'd finished my pizza and usually had to wait several minutes to grab the server's attention as I picked a poor choice of seat and they were a little busy.
You also get unlimited dough balls, which they can do vegan if you ask for it, they're meh, if you're hoping to maximise your enjoyment I'd recommend not getting them and just enjoying the vegan pizzas until you're full to brim, no room wasted on dough balls.
There are two vegan pizza options, a margherita and a giardiniera.
tl dr even on a busy night where they ran out of vegan cheese and the servers weren't "on the ball" (in enabling my fat ass) I still easily fit 4 pizzas into the time slot with change to spare, which was definitely a good deal, so I do recommend it and I recommend not getting frustrated at the thought that the slow service is pissing away your time because time is very unlikely your bottleneck.