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)