I see where Tyne & Wear is now, I checked climate and if I have it right the avg low is about 0C in the coldest part of the year, with max recorded temps getting almost to -20C? If so I would consider that a mild climate. The record lows would be for very short periods. I wouldn't be too concerned.
It is true that unless the pond is covered by ice there isn't thermal layering. But with ice cover there is, which is why ice forms at the surface. The water below the ice is higher than 0C. Because the ground is warmer 4C water will be found near the bottom of the deepest point.
If you build a shelter over the pond to keep it a little warmer, in your climate, you should be aware of something called ?Aeromonas Alley?. When water temp is between 5.5 and 16.5C the fish's immune system isn't working very well while Aeromonas and Pseudomonas bacteria grow well. So if the fish get an infection for whatever reason it can be deadly. If you can keep water temp above 16.5C that would be great. But if your shelter keeps the fish in the Aeromonas Alley it would be better imo to open the shelter and let the water temp drop to below 5.5C.
60 cm deep ponds with Koi are very common here in the US, even in extremely cold areas, day time highs not getting above 0C for long periods. Deeper is better for lots of reasons, but it is commonly done.