MPs typically move in packs ("areas" or "currents", that may or may not be officially organized around a thinktank or club), so buying the right MP you can actually move several of them. And then your representative goes and trades favors with the colleagues in government - "I'll vote for your controversial legislation if you pull that other lever over there". Representative democracy is a big sausage factory: in go blood and guts, out comes "edible" rules that society will live on.