As long as you are just going to the main theme parks (WDW, US, SW), you will okay with the UK one. If you are thinking about going to somewhere like Wal*Mart or elsewhere, then you would need the temporary visitors disable badge. We have only ever used our blue badge when we have gone out there, and no one has had a problem with it.
You do have to hold it up really high for the car park attendants to see as they are used to checking for disabled badges hanging from the rear view mirror. They also have to sometimes get a closer look at the UK one, especially if they are new to their role. However, there are always at least two or three British passes propped up in cars as we walk through the parking lot!