Check out this decision in Animal Defenders International. Particularly interesting (as Keith Ewing pointed out to me) is Baroness Hale’s opinion:

47. There was an elephant in the committee room, always there but never mentioned, when we heard this case. It was the dominance of advertising, not only in elections but also in the formation of political opinion, in the United States of America. Enormous sums are spent, and therefore have to be raised, at election times: it is estimated that the disputed 2000 elections for President and Congress cost as much as US$3 billion. Attempts to regulate campaign spending are struck down in the name of the First Amendment: “Congress shall make no law . . .
abridging the freedom of speech, or of the press”: see particularly Buckley v Valco, 424 US 1 (1976). A fortiori there is no limit to the amount that pressure groups can spend on getting their message across in the most powerful and pervasive media available.
48. In the United Kingdom, and elsewhere in Europe, we do not want our government or its policies to be decided by the highest spenders. Our democracy is based upon more than one person one vote. It is based on the view that each person has equal value. “Within the sphere of democratic politics, we confront each other as moral equals” (Ackerman and Ayres, Voting with Dollars, 2003, p 12). We want everyone to be able to make up their own minds on the important issues of the day. For this we need the free exchange of information and ideas. We have to
accept that some people have greater resources than others with which to put their views across. But we want to avoid the grosser distortions which unrestricted access to the broadcast media will bring. …

Buckley v. Valco?