It's .177, not .117. Is the smaller calibre rarer? They seem to appear in roughly equal numbers.
Unboxed post-War Seniors are worth between about £60 and £120 depending on the exact condition but generally represent unbelievably good value considering the fun that can be had, the quality of materials, and the workmanship. I have a boxed one in .22 cal that I've had for around 30 years and will never sell (though I would like to add a .177 barrel if anyone has one for sale?!). To me it represents the best example of Webley's overlever spring air pistol design, fused with olde worlde quality, before the slow slide began, via the Premier and Tempest, to the latest break barrel Typhoon, which is rubbish by comparison.