Notes: if you wish, you could boost the protein content by including some firm tofu (cut into matchsticks); for a low-fat variation, omit the avocado.
The Sushi nori, sushi rice, mirin, rice vinegar, pickled ginger and sushi rolling mats are all widely available in UK supermarkets or health food shops, but the cheapest place to buy them is likely to be your local oriental supermarket, if you have one near you.