Euphonium or Baritone?
I would have thought that there are plenty of secondhand Sovereign euphoniums around at the moment due to people trading up to Prestige (or Yamaha for that matter).
Seeing as you are based in Yorkshire then what about Band Supplies in Leeds. I'm a southerner so don't know of any other shops in your area.
However, Rosehill in Beaconsfield and Normans in Burton on Trent often have some interesting secondhand stuff.
John Myatt in Hitchin also have a high turnover of good quality secondhand stuff. Have a look to see if there is anything on this link;
http://www.myatt.co.uk/shbrass.htm
I got my 981 you see pictured from Myatts.
Every so often, you see good quality instruments on Ebay but obviously you can't try before you buy.