Just to say, UK mobile number transfers don’t happen immediately, @greekislandlover.
On the switching date, you’ll know iD Mobile have started working on your number transfer when your previous service provider SIM stops working.
It can take several hours or more for iD to connect the number (you want to keep) to their network, and then provision the number and network services to your iD SIM.
Once iD have started your switch, it can help if you restart the phone with your iD SIM on an hourly basis, until the switch has been completed.