The coin you have is a Churchill Commemorative Crown and NOT
halfcrown. Worth around 50p - £1 today (they were mass produced in
1965....just under 20 million struck in cupro-nickel - an alloy of
copper and nickel).
What is the value of a silver plated 1965 British Churchill crown pendant?
Modified coins have no collector value. Any value the pendant
may have would be as a piece of jewellery.
The coin itself might get up to £2 GBP if it was in mint
condition and had not been silver plated.