Here is a collection of pictures of Black-tailed Godwits, some of which are clearly islandica, but also some nominate limosa, the local form that sometimes breeds in East Anglia. The identification of Black-tailed Godwit forms is not as advanced as it might be (at least, not with me), so there may be some mistakes. Richard Millington points out that spring Icelandic birds should have some newly moulted tertials or inner greater coverts with heavy orange and black barring (thank you Richard). Mid-May is thought to be late for the Icelandic birds still to be in England, and so they would be immatures, late breeders or in poor health. This may be why, unlike limosa which has a distinctly orange-yellow bill at this time of year, they mostly have pink bills, even those in full summer plumage. Some of these islandica in full summer plumage in June and July have an orange-pink bill, but still nowhere near the yellow of limosa.
There are links to other Black-tailed Godwits here.