Is a (British) wedding cake just a tiered fruit cake
These days, wedding cakes can be pretty much any kind of cake. A traditional wedding cake is certainly more or less a Christmas cake - a dense, dark, 'matured' fruit cake with marzipan and royal icing.

However, nowadays you will find that anything goes - flavoured sponges are extremely popular. The main criteria is that the cake should be sufficiently dense to hold the weight of a tier or two. This means your standard light-as-air Victoria Sponge won't cut it - it will just collapse. You need to be looking at denser, moister cakes.

Basically, a wedding cake is whatever the happy couple want it to be!