Pour 150ml of the milk into a saucepan. Sprinkle over the gelatine and leave to soak, without stirring, for 5 minutes or until spongy.
Stir in the sugar, then set the pan over a low heat. Warm gently, without boiling, until the sugar and gelatine have completely dissolved, stirring frequently.
Remove the pan from the heat and add the remaining milk, the cream and orange zest. Scrape the seeds from the vanilla pod into the milk mixture, then add the pod too. Leave to infuse for 10 minutes while preparing the compote.
Place the rhubarb in a saucepan with the orange juice and sugar. Bring just to a simmer, then cook gently for 3–4 minutes or until the rhubarb is tender but still holding its shape. Spoon the rhubarb into a serving dish using a draining spoon. Boil the juice remaining in the pan to reduce it slightly until syrupy. Pour the juice over the rhubarb and gently stir in the sliced strawberries. Leave to cool.
Strain the milk mixture through a fine sieve into a jug, then pour into 4 moulds, cups or ramekins. Allow to cool, then cover and chill for at least 3 hours or until set.
To serve, run the tip of a knife around the edge of each pannacotta. Place an inverted serving plate over the top of the ramekin and turn them upside down, holding the two firmly together. Lift off the ramekin. Spoon some of the compote on the side of the pannacotta. Serve the remaining compote separately.