You can only destroy the key if it is outside the police control.
What you do is to put half the key on the internet and store half locally password protected on the encrypted drive (so both parts are needed to decrypt). To decrypt you enter the local password and the DNS or IP address of the second half. The second half could even be in plaintext even since it's purpose is to make the other key half useless not to protect the data.
Because the police don't know about this internet site they can't make a backup, so you can have it self destruct after a certain amount of time. If you can hold out for 7 days or a month or whatever then it won't matter if they have write blockers and clones, they will be undecryptable. Or better yet, since you're entering the internet address of the key you can give them one that does expire (a dummy cloud instance so you can log in and 'prove' the self-destruct mechanism), but you also posted the key to some random forum. Without knowing what forum and account there's no way to find the key, but if you actually get out then eventually and with enough TOR you can retrieve the key and get your data back.
Of course really you'll just spend the rest of your life in jail for contempt even if you can prove a self-destruct mechanism...