Today's additional upgrades:
Cupholders (naturally)

Bluetooth adapter with 3.5mm microphone input. I bought this from Amazon UK: https://www.amazon.co.uk/dp/B0BB2D4LRJ?psc=1&ref=ppx_yo2ov_dt_b_product_details

I recommend covering the gear stick and dashboard housing with towels to avoid scratches. I also noticed that I wasn't getting an 'Aux' option when pressing the mode button initially. I then found it's the white plug that needs to be removed from the original BMW wiring harness and placed directly into the back of the new harness that fits directly into the head unit (there are no through-connections on the adapter).

I then ran the wiring through the dashboard, up the 'A' pillar on the drivers side and into the overhead light housing. Using some 3M VHB tape I secured the microphone, with attached windshield, to the housing, aiming slightly downward to get a good clear pickup position.

To get the light out I used a plastic trim removal tool and released two tabs at the front of the light. Then popped it out, sliding towards the drivers side (there's a ridged lip on the housing to the left, which needs to go back in first - sliding to the passengers side, before pressing into place).

The finished head unit then looks identical to stock when refitted and has the bonus of clear hands free calls plus the automatic volume control which increases when driving at faster speeds and decreases at lower speeds, something that I did not have when experimenting with a non-stock stereo head unit.

When testing it appears the microphone is nice and clear for calls and others can hear me fine. It's good when the top is down, but at speeds higher than 40mph callers report a lot of background noise, making it harder to hear my voice. Therefore, if you really want a good phone conversation when traveling I would recommend either putting the roof up or shouting quite loudly (like you would when driving with someone else in the car with the roof down).
Overall, these are all valuable, inexpensive mods that will make my life my easier but keep the car looking and feeling stock.