This is quite an interesting topic, as it depends on a few different things. It is totally illegal to not display a front plate and have it on your dashboard, however if you're driving around in a Murcielago the police are quite unlikely to stop you for not having a front plate, unless you're already driving like a bellend, in which case the penalty will likely be added on. This goes for most of the UK, although some places like parts of Manc, Leeds, Bolton you might not be able to get away with it. There are plenty of supercar owners in the Home Counties area who never run front plates.
On the subject of side plates, largely illegal on the RS3 as you would have to run a smaller front plate than legally allowed. Side plates are only allowed on cars that are like that from factory like Alfa Romeos or Evos.
However, if you have a short personalised plate, you are allowed to run smaller plates that are totally within the confines of the law- the smallest plate you're allowed to run is 291mm x 105mm, which would be all plates 4 digits and under. Above that it's a sliding scale of minimum size allowed. At the same time, there are again plenty of supercar owners who run extremely small front plates and don't get pulled over.