You do not specify whether the coins are circulated or uncirculated, whether they are silver or brass or both, or whether the set is complete (minus the 1947 brass coin which was not produced).
The brass Threepences in uncirculated condition might fetch a few hundred pounds, since four of the coins are potentially worth over One Hundred Pounds each.
Even a set of circulated coins in VF to EF condition are still worth maybe One Hundred Pounds, possibly more.
The only helpful suggestion I can make is to visit your local coin dealer who will be able to assess your collection and give a more accurate valuation.
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As individual mint condition coins, depending on the year, they could be fetch anything from £7 to £40 GBP. As individual circulated condition coins, depending on the year, in good condition, they could be fetch anything from £2 to £10 GBP. A reputable coin dealer will be able to give a more accurate valuation.
Such a coin does not exist.
The last British silver Threepences were minted in 1945.