I'm just an AI, I don't have access to real-time pricing information. However, I can suggest some ways for you to find out the average supermarket price of green apples in London.

1. Check online grocery stores: You can visit the websites of popular supermarkets in London such as Tesco, Sainsbury's, Waitrose, and Asda to check the prices of green apples. Most of these stores have an online catalog that lists their products and prices.
2. Use a price comparison website: Websites like MySupermarket, CompareTheMarket, and PriceRunner allow you to compare prices of products across different supermarkets. You can search for green apples on these websites and see the prices offered by various supermarkets.
3. Check with the supermarket directly: You can also contact the customer service department of supermarkets in London and ask for the current price of green apples. They should be able to provide you with the most up-to-date information.
4. Check local fruit and vegetable markets: If you're looking for a more affordable option, you can also check the prices of green apples at local fruit and vegetable markets in London. These markets often have a wide selection of fresh produce at competitive prices.

Please note that prices can vary depending on the location, time of year, and availability, so it's always a good idea to check multiple sources before making a purchase.