Victoria sponge recipe relies on air for a light raised cake. Weigh your eggs and match their weight with the other ingredients, you can use up to one egg weight less in caster sugar. Sift the flour and 1tsp baking powder twice.
Preheat oven before you start, only drop the temperature if it's fan assisted. Bake in the middle of the oven. Keep oven door shut until 5 minutes before the shortest estimated cooking time and check cakes then.
Make sure you cream your, not too soft, butter and sugar until it is really pale. Add a tablespoon of flour in before you add eggs.
Beat your eggs with a couple of tablespoons of milk before adding them, one at a time.
Alternate with the flour and mix until the flour is just incorporated.
Preserve as much air as possible, it will look like the mixture is greater in volume and prevent flat cakes.
Hopefully some of that will help 😊
ETA I only ever use real butter, margarine adds oil which is not needed in a Victoria sponge.