Typically, companies don't give out information that they don't need to by law (other than when it makes them look good, such as charitable giving). Giving anything away is helping the competition, so the competitors spend time and resources trying to find out what each other is doing.
You may be able to get some information in press releases about new campaigns, but that will only be a proportion of the total budget.
How about approaching the Chartered Institute of Marketing and aks them typically how much large companies spend on advertising as a proportion of their annual turnover.
This article may help give you a clue - it suggests 5 - 20% of turnover.
a quick google has found a lot of answers through various financial papers and other interested organisations. Just google BP budget and you will find some answers.
At the end of the day, how much they budget to spend on these things is between the company, their suppliers and the taxman. It's nobody else's business and that's why you can't find it easily. If they don't want to publish it they aren't obliged to do so.