Now, that I've a good time to play around with it, Igraine being the daughter of Amleth/Hamlet and granddaughter of Cunedda Wleddig presents a few ideas:
Igraine would be Half-Briton and Half-Jute. In turn Arthur, Blasine, Morgause and Morgan have Jutish relations, adding to more to the heritage that already includes Bran the Blessed, Joseph of Arimathea and Brutus of Troy somewhere depending on the continuity.
This is especially significant in the case of Orkney Siblings, as Morgause' husband, King Lot, originally had a claim to the Norse Throne, while other times Lot is of Pictish influence, nevermind how Lot's kingdom was moved from Manaw Gododdin to Orkney. Multi-ethnic Gawain and co. ahoy!
As Amleth had two women from Britain as his wives, "Gwen ferch Cunedda" could be identified as one or the other. The most likely candidate for Igraine's mom is the Scottish Hermuthrude, as Cunedda Wleddig, although not exactly "Scottish", is from the "Old North" - (Manaw) Gododdin specifically, the area of which Edinburgh is the chief city and is, in Arthurian Lore, the location of Lothian, the original country of King Lot. It would be easy to imagine Saxo Grammaticus (1150-1220) simply labeling anything north of the Hadrian wall as "Scotland", given its borders in Saxo's time.
Hermuthrude as Igraine's mother - on top of the hilarity of being King Arthur's violent grandma - puts a rather dark and sobering context to Igraine's story and marriage status, if you know how Amleth's story ends.
Igraine being the granddaughter of Cunedda of Gododdin may explain her retreat to Canguin Rock - located in Galloway - after the death of Uther. As this was in the area of what would be known as Strathclyde later on, Igraine could just be moving back closer to her homeland.
Moreover, Igraine being of Northern Brythonic stock means the marriage of her daughters to Lot and Urien can be seen in a different light - its just bringing Morgan and Morgause under the custody of a territory and culture Igraine trusts and strengthen tribal ties to her native land.