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I guess your first job would be to find out the date of manufacture. If it is made before 1898 it would fall under the antique category. Below is a link to some legal mumbo-jumbo. After reading it I still don't know if it is legal with the short barrel. Maybe you will have better luck at understanding it.
I looked at that gunbroker ad and the seller has "no international shipments" on it so that will be a hurdle for you. There was a similar Webley MP on CGN within the last year. I think it he was asking $2,200 and it was an antique.
I have the same gun in an earlier model and it is a prescribed antique by year of manufacture and caliber. I legally imported my revolver from the USA without any issue. Barrel length is not an issue with antiques as I understand it. No 12(6) required.
I believe that the gun in the ad is also an antique as it says Webley and Son on the top strap. I believe that all Webley and Son firearms are pre 1898. I am far from an expert in antiques so I stand to be corrected.