As you’ve probably realised from the lack of replies, finding second hand, good condition tubes is nigh on impossible in the UK. Few shoot the gauges here and those that do hang onto their tubes for a long time. You may be able to find a 20g set as most that venture into the guages start with that.
Contact BSSA, they may be able to help. Otherwise try NSSA in the US as that's where the market is for them. Companies like Paxton Arms and others may be able to source some but you've got the expense of postage and taxes.
Failing that, several UK NSSA skeet shooters head over to the US for the World Championships each year and will probably bring some back for you. Expect to pay a lot of money though if it's Ultralites you're after.