Ive been a baker for 20yrs and since moving here, have never had so much trouble trying to make a cake...lol...i thought i was going mad at first, so started searching the web for the answers and came across a site...English baking in the Netherlands..or something like that...the first page of the site said "If you are English and going to make a cake using your recipes...forget it...they wont work"...and she was right.
Your flour and uk flour is totally different, your self raising flour is not the same as ours an ingredient is missing in either your or ours..your plain flour makes UK pastry recipes a very hard texture. Also your sugar is totally different to ours...your fine sugar is similar to our caster sugar so thats ok, we always make cakes with caster sugar not normal sugar and definately would never use basterd sugar as its far to heavy. We would use that for making the heavy fruit cakes we make at xmas. Your normal white sugar is heavier than our normal sugar we put in tea and coffee, we hardly ever bake with it, and as your normal sugar is heavier it will give a firmer texture to cakes if you use it.
We have 3 types of baking flour...self raising...plain and cake flour.
Cake flour makes cakes very light and airy, and self raising flour does and excellent job also, we never bake with plain flour for cakes unless you add a teaspoon of baking powder, but lol...your baking powder is different also...
So trying to convert an English recipe will take lots of tries to get nearly the taste you want, and if you havent tasted an English sponge then you wont know what it should taste like. Your biscuit mix i was told is like uk sponge, i dont think it is at all.
But if you do convert an English recipe and you like the taste then thats good, but it wont taste like it should do.
Hope this helps somewhat...