Melted vs softened butter in cake recipes
Welcome to the site! This is a good question, the answer is that there would be no difference in taste, the difference is in the texture of the sponge. Victoria sponge, and some other types of sponge cakes get some of their their lift (expansion when bakes) from air trapped in the batter. Beating softened butter and sugar together, which is called creaming, traps air in the mix, giving the cake more lift and a more open structure. Mixing melted butter and sugar does not trap air, so your cake will be more dense and less open. 

You can cream butter and sugar by hand with a whisk instead of a mixer, it just takes more time and a lot more effort. If you don't have the money or space for a stand mixer (or just don't want one) a handheld stick blender with a whisk attachment is a low cost and minimal space alternative which works extremely well.