Isn't it just.
There is a nest in Snowdonia with unringed birds and more in Scotland and the Lake District. Plus you have many birds that are travelling to Scandinavia via the UK. Birds that have been born in UK and unringed will also be coming back and looking for new sites, hopefully the new nest site set up in Dorset will attract new birds that have no breeding site, plus I know that they are hopeful that Hampshire will attract a pair in the not too distant future.