I got the assignment to contribute something about the status and distribution of the nominate Lesser Black-backed Gull here in Sweden.
That is not really possible without discussing the other subspecies, too.
I base this largely on an article in a Swedish birding magazine from 2011:
Vår Fågelvärld vol 70 nr 2:
David Erterius: Olika typer av silltrutar i Sverige (Different types of Lesser Black-backed Gull in Sweden)
Warning: it gets messy!
I also scanned some pages from a field guide which of course has to try to make sense of the mess.
Luckily for me almost all the birds in the Stockholm area look like pure fuscus.

Larus fuscus fuscus breeds mainly around the Baltic Sea, in Finland and in the White Sea in northwesternmost Russia.
It is a long distance migrant wintering mainly in tropical eastern Africa.
Contrary to the more western populations graellsii and intermedius, nominate fuscus is declining.

Nominate fuscus is the breeding Lesser Black-backed Gull along the Swedish east coast (i.e. Sweden's Baltic coast).
It is replaced by intermedius along the Swedish west coast. Intermedius also breeds along much of Norway's coast and south via Denmark and northwestern Germany to the Netherlands, where it meets graellsii.
Graellsii breeds mainly in western Europe from Portugal north to the Netherlands, in Britain and Iceland. In recent years it has been expanding its range. There are now sustainable populations on the Canary Islands and in Greenland. Breeding colonies of Lesser Black-backed Gull on the Swedish west coast contain more and more light mantled individuals, some of them as light as birds from the westernmost graellsii-populations.

In the graellsii-intermedius contact zone in the Netherlands and northwestern Germany intermediate birds, sometimes called "dutch intergrades", are found. These birds' mantle colors form a continuum of gray tones between the parental populations. Presumably birds from this area spread and reach i.a. southern Sweden. In breeding colonies along the Swedish west coast birds with all kinds of shades of the mantle color can be found nowadays and light mantled birds are more and more often observed in the southern Baltic.

The situation gets even more messy by the fact that Russian heuglini-type birds regularly reach Finland, which is not far away. Heuglini breeds in remote areas in Russia which are not normally visited by birders or banders, so there is no hard evidence in the form of banded heuglini being found in Finland, but the light mantled birds regularly observed in Finland look like birds photographed deep in the breeding area of heuglini in Russia. Also, in the Netherlands banded intermedius have been found that they thought looked a lot like the heuglini-type birds found in Finland, and banded graellsii and intermedius have reached Finland.

Here come the pages from the field guide, rather big jpgs (around 7 MB each).
They are from the highly recommended Collins Bird Guide by Svensson, Mullarney and Zetterström.
Their page on Great and Lesser Black-backed Gulls is here.
An illustration comparing juvenal Lesser Black-backed with juvenal European and American Herring Gulls is here.
Their general introduction to gulls including an introduction to age-groups is here.