In my opinion, Drayton Manor could easily sell more items of ride related merchandise on the on-ride photo booths at the exit to some of the rides. Or at least signs saying ride merchandise is available in the shops. Considering the main gift shop in the park that sells park merchandise is branded as “The Toy Shop”, it’s not the most likely place theme park fans would want to go into, unless they knew some things were sold there (behind all the toys of course).
As for “Giftbox”, the shop you enter after exiting The Haunting, it has no merch for the ride, which is very stupid considering it has a great logo that’s easy to stick on a mug or a magnet.