Prescription formula ££ uk

My baby has finally been diagnosed with cows milk allergy, the hospital gave us 2 tins of formula to come home with and wrote a letter to his GP asking for a repeat prescription for it, but we can't get the prescription till Monday and his last tin of formula will be finished by the end of today, so I took the copy of the letter to the pharmacy as proof and asked to buy a tin, it came to £18 for a 400g tin which is crazy to me because that's double the price for half the formula we used to get before (it was cow and gate, £9.50 for 800g).

Does anyone know if it'll always be £18 or will the price drop when we get an actual prescription? I don't mind paying that much if we need to, I'd rather that than all the problems his been having and now possibly a surgery depending on his ultrasound on Monday because of this allergy.