The process is to report the phone lost or stolen at the time, iD Mobile will blacklist the handset, block the sim and send out a replacement. Why did you not do this at the time of the theft?

Because I’m very busy, and needed a working phone ASAP and was unawares of any ‘process’. I went to the shop, got a new SIM and registered it. AFAIK, blacklisted phone locks can be bypassed, and iCloud is pretty robust/secure at blocking phones for further use.
Why do you hang around on a phone forum being a condescending, patronising k******d? Seems like you’re trying to elevate yourself to compensate for something.

I was just curious as to the reason why you requested a replacement SIM but didn't report the theft at the time because both options require you to speak to someone at iD Mobile.

Anyway, speak to a live chat agent, explain that your phone was stolen, provide them with the date and tell them you want the handset that was stolen blacklisted but explain that you have already received a replacement SIM card, and they should be able to blacklist the device without blocking your replacement SIM in the process. I would only mention one device to start, otherwise you might cause confusion and that's usually when things go wrong in a big way.

https://idmobile.co.uk/live-chat