What do you mean by safety here?

It's not safe in cases you are in some blacklisted person that British government can confiscate your assets from. Or if you are afraid to become this blacklisted person in the future.

It's safer than storing then safety custody, because they have way more security behind them then you could ever provide. But just acknowledge the fact that they aren't holding bitcoin either. So you aren't buying bitcoin, but synthetic representations of bitcoin.

In short, you won't be able to withdraw that bitcoin, you are just able to cash out the value that's gained, or lose the value in case it drops.