They can be out side full time when they're fully fledged.
Wait til 3 months to let them swim un aided!
and they can be out druring the day while mostly fledged! (as long as it isnt to cold or wet!)
What breed are you hatching?
Aylesbury are very fast growing ducks you may find they will have to go outside at around 6- 8 weeks purely because of their size. My 6 week old runners can easily get in and out of a kiddy pool on their own- but my Aylesbury Drake cant even get himself into a babies bath he is so large. Having a container with low sides is important for such a heavy breed of duck.
I will let mine outside when they are fully feathered - we do get buzzards here though and they have a habit of picking up the young... even our geese are not allowed out unsupervised until they are fully feathered.
Luckily here Spring seems to have arrived - most days it is about 17 degrees.... good luck in England with Spring..... from our experience of having emigrated here 3 years ago from Southern England we have Spring much earlier than you.
By the way, ours do swim when they are fully feathered 8-10 weeks of age without any problems - they follow the ducks and geese into the water and they are off....!!!