Hi Alice,
I'm an American, also living in Florida--Ft. Lauderdale area--I've never heard of Bero flour, but I do know that some cities have 'British shops' which carry items you're used to from 'home'. I tried to find this info online and arrived at a general site for 'Brits in Florida', which I thought you'd may find helpful. It includes information on many items/subjects:
http://sunnybrits.com/british/resources/
I know that sponge cake has to be very light, and did you buy 'all purpose flour' or 'self-raising(or rising) flour'?
If you're using all purpose flour, you'd have to add 1 teaspoon of baking powder per 1 cup of flour. If you're using the self-raising, the baking powder is already incorporated, and that's why it would be recommended for a sponge cake. I would suggest that you look in your local yellow pages and see if you can find a British grocer or gourmet shop in the area. Even if they don't carry Bero, perhaps they know where you can get it, if the self-rising flour is not what you need for your sponge cake.
P.S. I don't know why this site won't allow use of the word 'tea spoon' (with no space), but it keeps coming up as 'teathingy' as you can see! weird! lol
Good luck!
Cher : )