How much is a half US dollar coin worth in sterling?

It depends on the date. If it's dated 1971 or later it's an ordinary circulation coin worth one half of whatever the current USD-GPB rate is. For example, if the current rate is £1 = $1.60 that means a US dollar is worth about 62p, so a half dollar would be 31p.

Half dollars dated 1969 and earlier contain silver so their value is dependent on the current price of silver rather than their exchange rate.