I think waterlogging is a lot more to do with poor drainage than lack of evaporation. Evaporation will be a lot less in a cold damp UK winter than in summer even without a mulch.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I think that concrete 'pan' presents problems beyond winter water logging - especially if it covers a large area. It will probably be the reason the soil doesn't drain in winter and dries out so much in summer. Tree and shrub roots also won't penetrate it and you'll always be restricted to shallower rooted plants - which are often more affected by soil moisture levels.
You may need to investigate how large the pan is and perhaps give some thought to breaking it up. That would both aid drainage in winter and would also give plants access to deeper ground water in summer.
Finally, if you're resigned to living with the concrete pan, you'll find some of the hardy geraniums are more forgiving of the extremes of wet and dry. So long as it isn't sopping wet they'll cope with a bit of damp through winter and (with a deep spring mulch of organic matter on the winter-wet soil) they'll cope with a bit of summer drought.
I wouldn't consider breaking it up. That would be both too much effort and/or expense, not to mention the serious damage that might be done if the concrete is covering drains etc. Instead, I'd simply remove those plants that survive, gravel over the bed and stand lots of pots on it. Time will tell....
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