As a boxlock, sold by Woodward, it was very probably made in Birmingham by W & C Scotts, Midland, or another. If so it will be a good sound gun, but probably not 'made' by Woodward.
How that works in 'brand value' and 'original quality', I have no idea. I don't know how those charts are meant to apply to guns with a big name, but probably more humble origins (as most big name boxlocks are).
In my view, it's value is what it makes at auction, and so Holt's estimate is the guide as it stands now.
IF it was sleeved, (again in my opinion) any increase in value would not cover the cost of the work.
The only reason I can see to sleeve would be if the gun had sentimental, or family heritage appeal.