Regularly? I can only recall two instances of such. Both involving UK citizens.
There have been more than that. Don’t you remember the Ukrainian president they poisoned that basically kicked off their whole designs on the Ukraine?
Actually, now that I think about it, the Ukrainian was a dioxin poisoning, which is almost as nasty as Polonium poisoning. But yeah, they use poisonings regularly, with dioxin and polonium being favorites perhaps because they don’t give a quick and painless death but a very nasty one.