Get Doves Farm gluten free flour from the free from aisle at Tesco/Waitrose. Make sure your baking powder is wheat free. Tesco bp is fine.
Dairy free baking margarines are available - I usually use Tomor from the kosher aisle, but Tesco no longer stocks it.
Stick with a regular 8/8/8/4 recipe but add a little extra liquid (I add lemon juice and zest from 1 lemon) and get as much air in it as possible. I use an electric whisk to mix everything together and whisk until the batter is like a smooth, silky (but heavy) mousse. The batter will be thicker than a regular cake batter but works in exactly the same way.
Always add vanilla to the cake in addition to any other flavouring.
If you're making chocolate cake, make sure you replace flour with cocoa, rather than in addition to the flour.
The results will be the same as with a regular cake and no-one will be able to tell the difference!
With 8oz flour/sugar/marg and 4 eggs, you will make enough for 2 sandwich tins or about 24 fairy cakes.
Good luck!