I have recently moved to the UK and have always made all my recipes using American double acting baking powder with great results. I have tried the same recipes here using the regular baking powder from the supermarkets, however the results weren't that great. Can you advise how to substitute UK baking powder in my recipes?
UK baking powder is also the "double acting" type and we have in the past found that you can comfortably switch between UK and US baking powder (for Nigella's US books the baking is re-tested using Gold Medal flour and Davis baking powder and the results vs the UK ingredients are fairly comparable). We would suggest trying a good brand of baking powder such as Supercook/Dr Oetker. Baking powder does expire after a time so check the "best before" date on the container and make sure it is kept in a dry place. If you have moved into a new apartment you may also find that the new oven cooks slightly differently to your old one and you may find that you need to adjust oven shelves and/or baking times to get better results. If it is an old oven it may be worth checking that it is running at the correct temperature by using an oven thermometer - these are fairly inexpensive and can be bought at cookware stores and some department stores.
If you are still unhappy then you can try making your own double acting baking powder using cream of tartar, baking soda (bicarbonate of soda) and cornstarch (cornflour) in the ratio 2:1:1, so to make 4 teaspoons of baking powder use 2 teaspoons cream of tartar, 1 teaspoon baking soda and 1 teaspoon cornstarch. This mixture can be kept in an airtight container in a dry place for up to 3 months.