The post office is one of the few places that will accept them as legal tender(mine always has). I buy tuem and use them to buy stamps/postage anything else the PO sells.
I cant find the link, but there was a big fuss about a bloke who uses his coins to buy petrol from Tesco/Morrisons etc, he made the papers and recieved a settlement from thr police for being wrongfully arrested because Tesco wouldnt take his coin. Turns out theycall had to owe him an apology in the end. He still does it now.
**EDIT - These coins are "cash". They hold legal tender status having s face value and having been manufactured by the Royal mint.
I'm tempted to try this, if I have free day to deal with the police!!