Given a full collapse, wouldn't the UK resemble northern Canada?

There is a good way to see the problem in a second

Think about New York, is at 40 degrees N latitude. This means that if NY would be in the Atlantic coast of Europe would fall in Portugal! a little North to Lisbon! (38ºN). Sometimes Is easy to forgot that NY is "Southern Europe" league. Southern than the North Coast of Spain.

Now lets see Quebec (53ºN). Well, London is 51ºN. Without the sea current bringing hot tropical water, London should have practically the same climate than Quebec, and 90% of UK is higher than London, so would enjoy the full whole Canadian winter experience.

But the West Coast of North America is has milder winters than the East Coast, which has milder winters than the center. Shouldn't we expect London to end up with a similar climate as somewhere on the West Coast of Canada at the same latitude?

Probably Yes, because NY is also warmer that it should be. It receives still part of this warm water before the current turns toward east, plus it has also a heath-island effect created by cars and houses.

So if you remove this effect for both Europe and US, NY should be much colder than it is currently (And most of Europe also).

I think the hard part of this is predicting how the global jet stream and polar vortex winds might change in response to disruption of the Atlantic currents. It already changes for North America in response to the Pacific subtle temperature oscillation we call El Nino / La Nina.

The proximity to the ocean and the general westerly winds of the mid latitudes mean that the west coast gets a moderating marine influence. The cold ocean surface provides us "air conditioning" in summer and yet is also a mild "heater" in winter, helping keep coastal air above freezing temperatures.

But, this general background pattern is regularly disrupted by large high and low pressure systems that wander through the region and give us our weather of storms, cold snaps, and heat waves. These cyclonic systems can both carry a somewhat distinct air mass inside them and also disrupt the normal westerly surface flow. And the jet stream is largely in control of steering these systems, making them track further north or south or even helping them stall out temporarily in their generally eastward course.

> Shouldn't we expect ... similar climate as ... West Coast of Canada at same latitude

The Pacific ocean has a similar current to the Gulf stream (AMOC) called the Kuroshio current. If the Gulf stream stops, you should expect the climate in London to be worse than the climate in the center of northern Canada.

> the Kuroshio current

Everything I've read about the Kuroshio current only talks about its effect on Japan. Does it carry all the way to the North American west coast? Looking at this map[0], you can clearly see the effect of the AMOC on how warm the waters north of Europe are compared to similar latitudes elsewhere, but there doesn't seem to be the same relationship between the waters east of the West Coast vs. west of Japan. If anything, the Californian/Oregonian shores appear cooler than the same latitudes in Japan.

[0]: https://www.ospo.noaa.gov/data/sst/contour/global.c.gif

Yeah, there is a cold water California current that flows south along the coast, but it is the green band of water in that image (>16.7 c, >62 f) in the higher latitudes (>45" N) that is the warm water circulating. Both the Pacific and Atlantic have similar circulation, with Kuroshio current and the Gulf stream being the respective Western Boundary currents.

yes, likely even milder than that due to being also an island

but it's complicated, many other ocean currents exist and affect weather and wont stop

and basically only the overturning part of the AMOC is at risk of stopping, which means e.g. the gulf stream won't stop, but will likely become a bit colder and maybe a bit slower too

you can't really do comparisons like that as there are other currents which overlap with the overturning one and they won't stop even if that one stops due to them getting a lot of their energy from earth rotation. Through depending on which you look at they might become a bit hotter/colder.

Also geography making it more, or less, likely for constant wind from the inland affecting the wetter can also affect the normal expected wetter noticeable.

E.g. UK being a island means there are much less affected by inland wetter moving over to them then many typical Canadian comparison places. (And from-inland wind is the main cause of very hot summer or very cold winter wetter in many places e.g. cold weeks in north Germany, or e.g. the main source of Blizzards in the US).

So IMHO comparisons like "this city is on the same latitude and also on the cost so it will have similar weather" without the overturning current are pointless (which doesn't mean that they can't happen to be correct, but they as much can happen to be fully wrong).

I wonder if it would also mean that South America gets hotter, since that stream was carrying heat from SA to Europe