The northern rim had to be high for the lake to be deep and hold a lot of water. Low rim = low water-holding capacity = not enough erosion. And a high rim would be difficult to submerge and erode, and it would be right there as a very prominent underwater feature somewhere in the North Sea.
A lake can be broad and hold lots of water and a lake's edge does not have to be a prominent ridge to exist - just enough to keep the water in. After all the Gulf of Mexico is not contained to the North by a ridge, but merely a land mass which is above sea level.
I'd need to check the dates, but it could have melted. Ice bound lakes were present in the US as well, just in the last glacial epoch, check out Lake Agassiz or Lake Souris in North Dakota. The ice sheet changed the course of the Missouri as well.
Only relatively small relief beach ridges or wave cut terraces would have survived any shoreline on land, and those would be at an altitude reflecting the water level in the lake. That should be verifiable if it existed.
You wouldn’t necessarily need a large land rim (or any at all as far as I can tell) as the lake was ice dammed by a massive ice sheet. And besides, even if there was a rim, depending on how thick, it could have easily broken under pressure from the melted ice that went on to form the North Sea. And don’t forget about all the erosion that would have taken place upstream of the original breach/spillway. This probably would have made any rim even more unstable.
Example:
It is noteworthy that not only did erosion occur in the spillway below the dam, but above the dam as well (Figure 2). In the approach channel above the spillway dam, valley sediment fill and the top layer of shale (Holmesville Shale) were removed to expose and erode the Fort Riley Limestone.
http://www.creationresearch.org/crsq/articles/31/31_2b/31_2b.html