OP, if you have actually reported the theft to the police and they have given you a crime/incident number, you can use that number in communications to various companies in getting them to cancel accounts and put blocks on accounts. With regards to whatsapp, just contact customer support and tell them about the iphone theft and that your account has now been compromised by the thieves and that you request that your account be blocked. Then tell them the theft has been reported to the police and here is the crime/incident number. Because whatsapp operate in the UK they are legally obligated to act on your information. Customer support should ring up the police giving them the number you've given them so they can confirm that a) it is actually you the account holder and b) that a report to the police has been made. If whatsapp fail to act on the information you've given them you can take them to court if their failure to act causes harm (mental or financial) to you or to anyone in your whatsapp contact list.
So I suggest to write off an email to whatsapp ASAP, provided you've got a police crime/incident number because without that no company is going to help you.