[harrymac]

BSA

Made their Military pattern air rifle available to cadet and volunteer units in the early years of the 20th century. It was a BSA Standard underlever air rifle stocked up to resremble a MLE, IIRC there was also a shorter version available to more closely mimic the SMLE. They were even fitted with a dummy bolt handle. If you can find one, expect to pay in the region of £1000.
Webley's MkII air rifle was known as the "Service" air rifle apparently because some cadet/volunteer units bought them, although they were never officially supplied. The Webley was a very high quality rifle, but looked nothing like a service rifle.

 

[harrymac]

Google

If you do a google seach for BSA Military Pattern Air Rifle you'll get quite a few results, unfortunately I didn't see any decent photos of one. Pricey and desireable.
The Webley offering did indeed have interchangeable barrels, .177, .22, and .25. The additional barrels were extra cost items. The Webley MkIIs are also increasing in value but due to the fact that they were available for general sale to the (well off) public they are no where near as rare or valuable as the BSA Military Pattern.
Sadly all Webley airguns are now made in Turkey, and the familiar barrel over cylinder air pistol design has gone, to be replaced by a cheap in line design.