I had a 1.8 156 SW while I lived in London, it was nippy on rat runs and not too bad on speed bumps, it was a good cruiser too. I had to drive the 159 Sw down through Swiss Cottage, regents Park, back of Mayfair, back of Vauxhall/lambeth, back of Brixton, Herne Hill and Dulwich- it was a nightmare. Okay I hadn't driven in London for a while, but the car was all wrong for it, especially with the amount of speed bumps, it had me thinking if I lived there now, I'd have a Chelsea tractor.
Unless you're hankering after the looks of a 159, I'd get a 2.0 twinspark SW (not sure if you can get a facelifted TS), nice and revvy, nippy and it'll be cheap enough that you can throw it around a bit. I would say a 156 is a rat run car, a 159 you wanna be using the main roads, stay off the rat runs that are riddled with speed bumps.
GT 3.2 V6 - Q2, KONI FSD & EIBACH PROKIT, GTA REMAP, MICHELLIN PS3's, BMC CDA
159 SW 2.4JTDM TI - ALFATUNED, WIZARD BACK BOXES, KW V3 COILOVERS, NOKIAN WR A3/MICHELLIN PS3, PCA DYNAMICS ALIGNMENT.