I’ve owned and played some 10Ms and Super 20’s.
Conn: I like the ”Naked Lady” models (c 35-42). I had a silverplated Conn 10M # 287XXX with underslung octave key. A strong player. Nice tone and good intonation. Rolled toneholes. The ergonomic was ok. The other Conn I’ve owned was a 326XXX c -47-48. A standard 10M. Straight toneholes. Both were good, but I prefered the Conn from the 30’s.
King: I’ve owned three King Super 20s. A 296XXX (early 50’s) with underslung octavekey and silver double socket neck. Otherwise a standard Super 20. I think these Kings had hardsoldered tonholes. The other was a 425XXX (c -66) horn. A standard model. No double socket and drawn toneholes. This was a sax that had some in common with a SML. ”Both saxes are speaking the same language”: Big tone and also freeblowing! The third Super 20 I had was a -83. The model was called 2416k There was some trouble with that sax. The neckjoint was to small/wrong dimenssion. I have seen four of these models and they are all, more or less, suffering from this. This was a UMI horn. My sax sax was a 871XXX sax but they were not manufactoring after #800XXX, according to some sources? All three King saxes had they own characteritics. Hard to believe that they came from the same model. I almost got a feeling that they were just sharing the name? The best one was the 296XXX sax. Hard to get an even scale on my Kings. The last one was the best when it came to this.
Both Conn 10 M and King Super 20 became worse over the years. The same goes the Buescher (the peak was the 400 model). Among the four big American saxes Martin was an exception. The kept a good standard to the bitter end.
I can't help you with how to find a these saxes in London. But Tenor Madness use to have some in stock. I think it's better to buy one which is overrhauled and ready to play.
This is my own thoughts of these lovely horns. I regret I didn’t kept the Conn 10M #287XXX and The King Super 20 # 296XXX. Both are milestones!!
Thomas