Send it to Steve Bertram for a good clean and service.
That's all it needs.
DO NOT have Turnbull touch this rifle.

Edit: Sounds a bit harsh I guess, so I’ll explain myself.

Turnbull does a very nice job restoring and coloring American firearms - has that process down pat.
British guns aren’t their bread and butter work and I think (as many other collectors I talk to) they just don’t achieve the British style in color case hardening and they end up a bit too garish when done.
As a long time collector of UK single shot rifles, this particular rifle is, in my opinion, a good example of a well used but not abused, proper dangerous game Farquharson.
The original condition and surviving colors make me think that a good strip and clean including a function check, as these Farqs do have a couple areas which can be a little problematic, is all that is needed to get her looking good and functioning as she should.
I’m guilty of having a coupe British rifles fully restored but they were true project/parts guns.
A good original example is not all that easy to come by these days and yours is certainly a good one.