The pre-war Webley air pistols are probably the most desirable from a collecting point of view due mainly to their comparative rarity, but this also means they attract considerably higher prices when they do come up for sale. The flat spring clips you refer to were used to hold the barrel in place on the first (1924) Mk1s once the pistol had been cocked. It is believed a single spring resting against the barrel was fitted on the very first models quicky followed by a double spring arrangement and then finally replaced with the traditional catch as the spring tension sometimes made releasing the barrel difficult and also resulted in quite deep marks at the point of engagement with the barrel. No examples of the single clip model have surfaced and very few double clip variants have survived resulting in them being the most expensive. I would expect prices for the double Spring Clip Mk1 to be in the £1000-£1,500 price range if you could find one.
A bit more common but still rare and in order of cost would probably be the pre-war Mk2 Target, Straight Grip Senior, Slant Grip Senior, Mk1 and Junior. All of these attract prices in the £200-£500 range depending on condition. The post-war models are a bit cheaper, but still attract quiet high prices with the Senior and early Premier models being most desirable.
So, if you are mainly interested in pure collecting rather than shooting them, I would go for pre-war models, but the post-war pistols while still being collectable are cheaper to buy and better suited to regular use with spares being readily available for all these pistols. You might want to get a copy of this book before you start down the long but rewarding road of Webley air pistol collecting.
Brian