Honestly? Where are you shopping, what are you comparing to each other? I think the segmentation of stores in the UK and Germany are very different.
UK has Waitrose and (for some things) M&S for high quality fruit and veg. But they are also highly priced. Germany does not have a posh equivalent. However the common non-discounters (Rewe, Kaufland, Edeka, Globus), are really not bad.
I am not sure about the exact number of fruit and veg SKUs in Tescos and Sainsburys (UK) but I often find them of less quality than I remember them in Germany. I never had issues shopping fruit or veg in Germany nor the UK though.
I also noticed that the UK imports very aggressively non-seasonal products from afar. Like beans from Peru or blueberries from Morocco. I am not sure of that happens in Germany, too. So potentially you are noticing exotic fruit and veg having a different availability, which might be due to different customer awareness towards the impact on the climate of imported goods from afar.