Advise needed - Webley Senior or Premier?

i think you are going to go down a very deep rabbit hole and you will find that the answers you ask are going to have opinions tied to it
the Senior pistols are of a higher value then the Premier and the Seniors were made for the most part much earlier
now i suggest you join and post on AGF in the UK i think you might find some Webley knowledge there
enjoy your rabbit hole
https://airgunforums.co.uk/
 
Based on 40 years exp of them, neither is better. They have their own advantages and disadvantages.

The Premier is often spoke of being better due to its adjustable trigger versus the tough non-adjust 5-7lb pull of the Senior, but the Premiers piston is too heavy and makes less power than the piston ring of the Senior.
The Senior is marginally better made with no cost cutting measures.

My favourite is the Senior..
 
I, too, have owned 4 Webley pistols for more than 35 years...a Senior, a Premier, a Mark I, and a Tempest. Is a Premier better than a Senior or vice versa? Heck, I like all of them so much that it's really difficult to get that far down on the weeds. I will say that I have been able to reduce that 5-7 pull of the Senior trigger by extensive polishing the moving parts and using some dri-slide mixed with Beeman trigger paste on the meshing parts. It's still quite a pull.....I haven't measured it, but still probably around 3.9 to 4.4 pounds. I made a trigger shoe for mine that further reduces the pressure by spreading it across more of the finger. I don't consider either of these more than a serious plinker. All 4 of my Webleys are .177, though I have .22 barrels for 2 of them. I think you would enjoy owning and shooting either of the 2 in which you are interested.