Sorry I know the questions wasn't towards me but I wear 5.5 UK myself. I have a sem-wide feet and high instep
I take 5.5UK on Carmina Rain/Inca (sometimes 5.5UK or 6UK on their narrower Simpson/Alcudia)
When I first contacted Rezso for sizing info, he recommended me i should go with 39 (39.5 for U last since he said they are narrower)
I've heard Vass makes it a little bit and recommend big so I took a chance and went with 38.5 G width.
They came and fit perfect for me. Hope that helps.
UK5.5 = EU38.5 to EU39.
Once again in line with the following link: