What makes it "not interoperable"? That's a matter of the comms equipment on board. And a NATO nation already uses EriEye, and it's news to me that the UAE does not need to interoperate with the US.
The UK is a signatory to the NATO AEW&C Force. The current Sentries are not tasked by the UK but by NATO. The requirement is for there to be "seemless" capabilities between the NATO E-3A and the RAF's E-3D to make this tasking straight forward. Any downgrade in operational capability on either side would upset this agreement.