Technically, releasing guineafowl is illegal under the Wildlife and Countryside Act, which forbids the release into the wild of any non-native species of birds or animals.
A notable exception is made for pheasants and red-legged partridges, both of which have been established here so long that they are regarded for legal purposes as natives.
However, guineafowl do represent a special case, as the birds we have in this country are of domestic stock, and there is no legislation that forbids the keeping of free-range poultry, including guinea fowl.
Chickens are not native to the UK, either.
I can find no record of anyone being prosecuted for releasing a few guineafowl with their pheasants.
It might be a different matter if an estate started to release large numbers of guineafowl for shooting, but this is hardly going to happen.
Guineafowl may offer good sport in their native Africa, but not here.