Why are certain fruit & veg bought in UK supermarkets tasteless?
Non-native and out of season fruits and vegetables that are available in northern countries (e.g. UK, Canada) need to be shipped from far away and will be picked before they are naturally ripe (under the normal sun and heat and getting nutrients from the soil).

They will ripen in controlled environments (UV lights, maybe controlled atmosphere and temperature) to be able to be ripen on time during the long transit to be ready when they are put on shelves in your local stores.

In the UK, it is currently the high season for strawberries (I assume it is the same time as in Québec); compare locally-grown strawberries to imported strawberries and compare the flavour and aromas.

(anecdotal) A quick tip when buying imported fruits (and vegetables); keep them outside of the fridge as much as possible, and if planning to eat soon, place them in the sun for a little while and the flavours will get better.