What would the best Cornflour substitute be for a Pavlova?
Cornflour in that sense being just starch, you should be able to use many starches with decent results. So corn starch, wheat starch, potato starch, rice starch, etc. will work, no matter what name they are sold under (e.g. you are more likely to find "rice flour" than "rice starch"). Try to avoid starches that are high in amylopectin ("waxy" starches), for the pavlova you want fluffiness in the starch.

If you are willing to experiment and end up with results that are farther away from the original, consider trying random flours from gluten free plants. I don't mean the "gluten-free flour mixes" in the supermarket which are enriched with gums, I mean things like oat flour, chestnut flour, etc.