Ian Hislop and journalists from Private Eye spoke directly to a select committee on the issue of lobbying. Where they read out gifts the MPs on that committee had received.
https://www.youtube.com/watch?v=Fg3DsTuG7yY
The whole thing is worth a watch, but they do ask the question of what these companies think they're getting in exchange for gifts, or for inviting MPs out to dinner, offering them jobs, etc.