Does it matter whether imported supplies are provided from inside the GCC or outside?

There is no practical difference between import goods provided from outside the GCC and ones provided from within the GCC. The reverse charge mechanism applies for both of them.  For example, if a supply is imported from Saudi Arabia into the UAE, the reverse charge mechanism comes into effect. The registered buyer will have to pay the VAT for the supply to the authorities.

If a product or service is imported from the UK into Abu Dhabi, the process of VAT collection is the same. The registered buyer will still have to pay the VAT to the authorities.