I can't see the photo for some reason. But I'd be STAGGERED if it really was Hypostomus plecostomus. That species hasn't been in the trade in the UK for, maybe 20 years? I know it is a name you'll see in every aquarium book (often alongside photos of totally different fish, such as Hypostomus punctatus). But it really isn't common in the UK trade, and not even that common in the US, either.
Almost all the "common plecs" sold in the UK are species of Glyptoperichthys/Liposarcus/Pterygoplichthys (depending on which name you use). Pterygoplichthys multiradiatus, and Pterygoplichthys pardalis seem to be the most common. The give-away clue is the dorsal fin. Hypostomus have 1 spine and 7 rays in the dorsal fin, whereas Pterygoplichthys have more rays, typically 10 or more.
Regardless, none of these fish has a high resale value. Far better to find a trusted aquarist you like who is keeping an aquarium containing large community species, and give him/her the fish. At least that way you know your catfish is going to a good home.
Cheers, Neale