We have muscovies and they are my favorite breed of duck. They are quiet (quackless ducks) and they are nice/friendly. They can fly, as others have mentioned, but you can clip one of their wings once a year to prevent them from flying off and that's often a more practical approach than building a coop/run that's enclosed with a roof. If you're not comfortable doing this yourself, your vet can do it for you. Our local very does farm calls and can the wings when they are coming out to do shots on the dogs and other animals once a year.
Muscovies don't need specialty vet care monthly (like flea treatment, for example), but I do think there are requirements in England for how to store their food and water containers to prevent wild bird contamination/spread of avian flu. I'm not aware of those restrictions here, so I'm not sure what to tell you about how to setup your duck enclosure but I'm sure, being located in England, you will have more ready access to that information than I do.
OH--- and if you have 250 sq ft per duck in open yard space, you don't have to feed them, they can eat enough grass and bugs in that amount of space to feed themselves. Of course, if it gets cold, you may want to supplement over the winter. Where I live the climate is mild 9 months/year, but we do have to offer them feed in the winter months when the grass isn't growing. (Alternatively, you could save up grass clippings and grow your own worms, but all that is, as they say, another can of worms for another day)
Muscovies are not like other ducks, though, and they do roost. In fact, mine prefer to roost and get upset if that option isn't available. I don't know if this is true for all muscovies, but ours seem to like being close to humans and were happiest when their house was placed close to our house. We have a mated pair, and the drake runs predatory birds off (I've seen him do it), so having an aggressive drake is helpful. Ours has never been aggressive towards humans, in fact, he eats right out of our hand and is a joy to have around. Our female muscovy makes a great mom. I think you'll really enjoy having them.
They are GREAT at pest management, too. They love to eat bugs and we haven't had to spray our house for bugs since having the ducks outside.
The only negatives I can think to warn you about would be that our drake doesn't distinguish between female ducks and female chickens, and he thought it was his duty to mate with our chickens as well (which is dangerous--ducks and chickens don't have matching sex organs/orifices, so it can damage the chicken for him to do that) so we had to separate our ducks and chickens. I've read that's not an issue with all ducks, or even all Muscovies, so this may just be our drake's particular attitude.
I hope all that helps!
My kids love the chickens, otherwise I would get rid of the chickens and have only muscovies. I think they're wonderful. I wish you the best with them!