Good Day @chengjing ,
I supposed there are direct and indirect way for anyone outside of UK to really order from your store.
On a directly perspective, you can block them through some app perhaps.
On an indirectly perspective, people can use VPN(Virtual Private Network) which disguised their location and purchasing from your store.
Perhaps the only way to do so will be upon making payment through credit card, check the credit card country like in binlist.net
In another word, instead of blocking them, why not knowing them and perhaps you can setup a store for europe serving them?