It should tell you when it needs a service when you first start the car
Either :
On first gen - the mileage countdown in the speedo on ignition start
On 2nd gen - in the rev counter on start up (on my Dooper its 12000 miles)
If your mileage is still low by the time the car gets to 12/18 months old, speak to your dealership as they may still do the service under TLC for you.
To change your reg :
Change it with your dealership, so the service dept recognise the car
Change it with MINI Customer Service - 0800 083 6464 (they change it with MINI Emergency also)
And if necessary change it with MINI Finance (they will send you a form to sign)
Hope that helps a bit