Kawasaki GT550, carries its weight better (and didn't need filling up 3 times a day like a CX500C ;) )I think my favourite ever commuter type multipurpose bike was a lowly old Honda CX500.
Narrow is the name of the game for inside M25 antics. A well-ridden small bike such as the old Honda CB Two-Fifty will embarrass bigger machines in London traffic and will sip fuel.
Last despatch tool I used was Suzuki SV650 (non-S), but if I had to buy* a BMW for going into that there central London, then I'd just rag a G310R to death.
*if I had to buy a new one. I would never buy a new one, so it'd be a '77 R100RS like I had on loan before - I quite liked the hands in a washing up bowl effect of the RS fairing / narrow bars