This comes up regularly in Crosman modding circles, because there's no difference between the action of a 2240 pistol and the 2250 rifle, only the latter's gas tube is an inch or two longer, and they're basically like Lego with interchangeable parts.
There is no UK legal definition of what an air pistol actually is, only that if it's >6 fpe it's section 5.
Some say it's the 1997 short firearm definition 12"/24". Some say it's how the gun is used, i.e is it designed to be operated with one hand and look pistol like, not rifle like. Then you get into what it left the factory intended to be and what configuration it's in now.
So it'll be down to interpretation and the only way to be 100% safe is to comply with all above: I.e. if it looks like a pistol, left the factory looking like a pistol, or
has barrel <12" or OAL <24", keep it under 6 fpe.
So I reckon cut down a S200 as above or fettle a Ratty. Be careful of the latter if you value your free time, and be aware that you can't polish a poo... but you can roll it in glitter!