Boil milk and cream in a pan while whisking egg yolks and sugar in a bowl until pale.
Pour hot cream mixture over yolks and cook over low heat until the custard reaches 82°C.
Stir in glycerine if using and pour hot custard over crumbled Christmas pudding.
Churn mixture in an ice cream maker and freeze according to instructions.
Simmer whole oranges in a covered pan of water for 1.5 to 2 hours until soft.
Halve oranges, remove seeds, and finely chop everything in a food processor.
Preheat oven to 160°C and grease a 24cm loaf tin.
Process 450g of orange pulp with dry ingredients and eggs until smooth.
Pour batter into tin and bake for 60 to 80 minutes, covering with foil after 40 minutes.
Cool cake on a wire rack before removing from the tin.
Simmer plums, butter, sugar, cinnamon, and lemon juice until plums are very soft.
Remove cinnamon, blend plums until smooth, and pass through a fine sieve.
Serve cake slices with a scoop of ice cream and plum sauce.