Her Majesty's airport officials may have issues with ANY minor that is traveling without their parents - older uncle being there or not
I would repeat an advice from above - call the British Consulate and ask them what documents are required. You may never get asked for them - but you never know.
And because she is technically minor, give a call to the airline as well and check what they require in order to allow her to board an international flight with you and without a legal guardian - in the very least you will need a letter from the parents that they are ok with you taking her out of the country. It is not just the British immigration people that you need to be in the clear with; Delta may also have objections.