Audi state that the max gross weight of the car is 2.4 ton. Now it suddenly doesn't become a death trap at 2.41 ton, there are shades of grey before it starts to get unsafe. However in the eyes of the law, there are no shades of grey, if you are over the permitted gross weight, it's illegal as they deem it unsafe. In my opinion, 0.5 ton over falls well within the unsafe category and Audi would say the same (if they thought it could handle 2.9 ton safely, then that's what would be on the VIN plate).
Normally the traffic police and VOSA go by how low your car is sitting to whether it is overloaded or not, however they can weigh them if they feel like it. Normally it's vans and trucks that get weighed and they will look at the VIN plate to get the gross weight. If they are over, then the driver is in the poo. It doesn't even matter if they upgraded the suspension to cope, they will always use the manufactures spec and the same applies to cars.
The Audi with that load couldn't handle all normal UK traffic conditions safely, which makes it unroadworthy and unsafe. For example, would you really want to take that car on the motorway? The tyre sidewalls are at risk of overheating at anything >50 mph and when that happens, that's when you can get a blow out. Taking it slowly would cut no ice with the law and you can't say "don't worry officer, it's roadworthy, but only up to 30mph".