If Drayton Manor was to have an online shop, they would have to have a significant increase in merchandise over he next few years to make one worthwhile. There’s no point in just having a website just to sell loads of discounted merchandise and Thomas & Friends branded stuff - when the majority of items sold in Thomas Land can be bought from online sites like Amazon and eBay anyway.
I remember around 3-4 years ago when Blackpool Pleasure Beach began their online store; there was barley anything on there, however since then the park has expanded the merchandise range probably quicker than any other UK amusement or theme park. Drayton Manor would have to do the same or something similar, however (as I’ve mentioned on this site many times before) the park-branded merchandise clearly isn’t successful, with you being able to almost buy it all/ most of it for under £20.
A shop could work if old ride parts were sold; but if they were to do that I’d hope they keep the items to a reasonable price, as Blackpool Pleasure Beach have continued to increase the price of ride parts so much that I don’t see many of them as a worthy way to spend my money.