The trouble is, there's a difference -- albeit subtle and sometimes they overlap -- between party donations and lobbying. Anyone, and usually specific groups or individuals, can donate to a party. So for example, David Sainsbury donated a shed-load of money to New Labour, and Tony Blair gave him a peerage. There are hundreds of examples of donations equalling access, and it's very transparent, therefore easy to expose and legislate against.
Lobbying is more subtle, in that it's often employed by sectors rather than individuals. It also doesn't involve such traceable transations. It can range from wining and dining to writing pieces of legislation for ministers. That's a lot harder to clamp down on.
There's also an argument that lobbying is all part of the democratic process. Unlike party contributions, which are legal bribery and only available to the rich (and trade unions, but not their members). Lobbying can be said to be people simply petitioning elected representatives to consider a group's interests when making policy. I don't agree with this view, but a lot of people subscribe to it. There's a great Onion story entitled something like, "US people hire lobbyist to fight for their interests in Washington." They have it much worse -- in terms of contributions and lobbying -- in the US.