I'm thinking of buying a cheap Galaxy S3. The phone is unlocked.
I was planning on buying a SIM only 1 month rolling contract from the network 3 to put in the phone.
The phone is currently running ICS version of Android. Am I going to have issues upgrading to Jelly Bean because the phone doesn't have the carriers 'front end' on the current OS?

Also are there any other limiting factors to putting a SIM in an unlocked phone rather than one that was designated for the network it is being used on?
Re; "Front end" When I read about Jelly Bean being released for the S3 in the UK i read an article saying it was acutely delayed by Samsung due to them putting their finishing touches on the 'front end' of the OS (I assumed they were referring to Samsung logos and stuff?)
So my question now is:

Would I have to use a custom ROM to update an unlocked phone?