To be honest you could do the same thing yourself quite easily.
The link that Creepy has posted above has more or less the same kits in it as the BMW kit would have been. The only challenging bits are hard wiring/splicing in the power and trailing the aerial wire.
The simplest install I could think of would be to put the DAB unit in the armrest (assuming you have an E92 here, I can't remember what the E90 has in the armrest) and power it straight from the 12v with a car adapter (so that it's all hidden.) Connect the DAB output into the 3.5mm aux jack and then wire the aerial into the back compartment under the carpet and onto either the back window or side window (depending on what works best.)
Then use the remote to change station and store it wherever you're happy with. You can then use your idrive/head unit to switch to aux and the DAB remote to work it. You should be up and running for circa £100 at retail rates (you can usually get a cheaper deal if you shop around.)