I have never heard of dessert grapes being grown commercially in the UK then (or even now). SLB shows his (lack of) age as that use of air freight was many years in the future!
My memory of the period just after the war is the gradual restoration of luxury (i.e. imported) foodstuffs in the shops - we had a farm during the war so I didn't see the fruit shops because we grew our own. However my parents had been (fruit-)farming in South Africa previously and one of their neighbours sent us food parcels during the war which could have contained some fresh fruit (robust things such as oranges) but were mostly dried fruit. It is quite possible that the grapes Franek is interested in came from South Africa, in which case I suggest they'd be in UK shops in March-April.