Divide all the food and non alcohol total in half and you get 23.25 GBP discount, so looks like that's what they did.
(coming from someone who had like 6 diet cokes over the course of a hot day today in London all at 50% off!)

OT: Wow, service charge and cover charge? Now thats a bit too much for my taste. And has anyone noticed restaurants adding 'optional charity donations'? I have been to 3 restaurants in London past 2 weeks with it, while before COVID I never had this issue in these establishments.

Another thing that interests me and please check this in T5 to confirm. The price difference at Pret a Manger on Kings Road between eat in and take out is 20% like the old days adding the VAT, even though Itsu reduced it in line with the VAT to 5% difference which it should be.
Is Pret a Manger profiteering here and pocketing the difference? Any suggestions welcome!