Here's that list of UK Skits:
Season 1:  
 
4. Ruth Buzzi: 
Song: "I Never Harmed An Onion", Rowlf. 
6. Jim Nabors: 
Song: "Another Case Of Dog Eat Dog", Baskerville & Rowlf. 
 
12. Peter Ustinov: 
Song: Song: "(Hey Won't You Play) Another Somebody Done Somebody Wrong Song", Miss Piggy with Fozzie, Scooter, Gonzo, Janice, Mildred, Hilda, George & Rowlf. 
13. Bruce Forsythe: 
Song: "I'm My Own Grandpa", The Gogolala Jubilee Jugband. 
 
20. Valerie Harper: 
Song: "*** Willow", Rowlf & Sam. 
22. Ethel Merman: 
Song: "Don't Sugar Me", Miss Mousey in Waldorf´s teacup. 
Season 2:  
 
48. Cloris Leachman: 
Sketch: "Vedgetarian´s Hospital"(Pigs)!!!. (has German subtitles)
Season 3:  
50. Leo Sayer: 
Song: "She Was A Dear Little Dicky Bird", Gonzo. 
56. Loretta Lynn: 
Song: "Gonna Make A Sentimental Journey", Gonzo & Kermit on lorry. (has German subtitles)
61. Helen Reddy: 
Song: "Beethoven´s Sonata No.8, Opus 13", 
Rowlf plays Beethoven, while the bust of Beethoven falls asleep. 
67. Elke Sommer: 
Bobby Benson's Baby Band plays. (I think; it features a baby voiced by Jerry doing some scats in the middle of the song)
69. Roger Miller: 
Song: "Rum", The Muppets in a bar. 
Season 4:  
75. Shields & Yarnell: 
Song: "Lonely At The Top", Gorilla on building. (has German subtitles)
78. Linda Lavin: 
Song: "Frog Kissing", Robin & The Frogs. 
86. Phyllis George: 
Louis Kazagger interviews The Flying Zucchini Brothers. 
 
90. Christopher Reeve: 
Song: "Sam's Song", Floyd, Jannice & Beauregard play and sing to Sam.
94. Andy Williams: 
Backstage: Kermit & Sam (Sam's family)