Makes no difference where it is sleeved, but a British gun that's been sleeved is out of proof unless it is reproofed at the proofhouse. No question that quality sleeving can be done, and you can be quite safe, with guns that have not been reproofed. However, if you decide to sell the gun to a knowledgeable buyer, it's going to take a hit in price for having been sleeved in the first place (compared to one with sound original barrels), and a second hit for being sleeved without reproof. Same story for a Brit gun that has had chambers lengthened without reproof, if you sell to a knowledgeable buyer. But of course that's a far less expensive modification.
Note that I have nothing against sleeved guns, or against sleeving one if you choose to do so. I've owned a couple myself, both British, both marked "sleeved" and so reproofed. Cost me a good bit less than if they'd had sound original barrels. So a gun with a good sleeve job can be a good buy, as long as the buyer recognizes that sleeving decreases the market value of the gun vs one with sound original barrels.
