There are quite a lot of complicating factors:
most but not all coins containing silver were hoarded or withdrawn much earlier
the craze for 'modern' began in 1966 with the government announcement of impending decimalisation and the 1967 date freeze : obvious rarities / scarcities were taken out by collectors, though many scarce varieties didn't come to light until after the event, and this applies to all denominations
halfcrowns and halfpennies were demonetised in 1969; most 1925 and 1930 halfcrowns had long gone, the scarcity of many Unc 1950s halfcrowns only emerged quite late, there would have been few around. Apart from those, they are both denominations notably lacking in rarities
florins, shillings, and sixpences carried on in circulation until 1992, 1990, and 1980 respectively; probably nearly all still in circulation got withdrawn - higher % than for 1971 I would say as there would have been little interest (and 'of interest') by then
only pennies and 3d bits were withdrawn in 1971; probably the majority of 3d's were withdrawn as the scarce dates would have been collected. This leaves pennies which as always leave more questions than answers
finally an unknowable but probably quite substantial (judging from the many rubbish lots at W&W in the 90s) quantity of postwar predecimals was saved in tins, pots, jars, Whitman folders, cheap albums, boxes, and god knows what else, as keepsakes by non-collectors.