Hello @Anonymous and a Warm Welcome to the Community Forum.
When you get your new SIM card remember to call O2 again to have the block you've placed on it (as adv by Damien1) to be then removed as your new sim will not work until this has been removed.
Make a note Of the time and date and agent you spoke to to advise the phone and sim was lost.
Lost handset O2 link ;
https://www.o2.co.uk/apps/help/help?qid=1&q1=2&route=lost_stolen&case=Lost/Stolen%20FormIf your phone did not have a pass code on it then I would suggest you change any passwords / pass codes that the finder may have got access to.