I stand to be corrected but I don't think the topography (I think that's the right word) and size of Tristan would allow the construction of any sort of airfield.
Wholeheartedly agree that overseas development money should be spent on our dependencies. I also think that there is a fair economic case for this anyway. We currently spend some £26m per year on St Helena and the airport is expected to reduce that as other forms of revenue increase.