Sleeving is readily available in the UK through any gunsmith but nearly all goes to John Foster or a firm just north of London who has been the leading light in developing the TIG method of sleeving as opposed to soft solder. Turn round is about 3-6 months in reality.
However, a friendly warning, if the gun is not pre-1897 you will be opening a whole can of worms if you send it over to the UK and want it sent back (rather than carrying it both ways). You would legally need an export permit from DTI which although free does involve a fair degree of paperwork and might need a customs broker to sort it for you.
