Here’s the press release details from my local plod:
People are invited to come along to the mobile police office in the car park and officers will register items for them on immobilise – a property registering database. You don’t need to bring the items with you, just bring the details of them and an officer will open your account which you will be able to add to at any time.
It only takes a couple of minutes to use the site to record the make, model and serial numbers of property from mobiles, iPods, TVs and laptops to bicycles, jewellery, power tools and portable sat navs. The secure site can only be accessed by the registered user.
Police check any items they recover against the database; if it’s been registered they can easily identify the owner and return the property to them – even if it hasn’t been reported stolen.
— http://www.northumbria.police.uk/news_and_events/media_centre/news_releases/details.asp?id=81352
So all they are doing is helping you register stuff on that website.