I would recommend Western Steam for a copper boiler but like all good things you will have to wait maybe 10mths. Helen did my 2" A7 Fowler boiler
http://www.westernsteam.co.uk/
You are not a million miles from Maidstone Engineering/Maxitrac they have someone who does welded copper boilers, supposed to be quicker and cheaper than traditional flanged/silversoldered
http://www.steamtechnology.co.uk/
Jason