Expenditure over £1,000 | Greater London Authority

From the page:

The Mayor is committed to providing financial transparency and has instructed that regular reports should be published on all GLA expenditure over £1,000, including VAT.

These reports are available in both comma delimited format (CSV) and portable document format (PDF), and are listed chronologically (by month) in the table below:

Around 30 csv files (running from April 2008 to the present).

Though no license is specified would assume this data is freely available for reuse given the statement on the web page.