The figure is 7.9 per cent of total UK ad spend - up from £287m this time last year.
The report also suggested that the Rugby World Cup helped to push up advertising revenues this year, with £3.9bn expected to be shelled out on ads this year.
And although television ads continue to dominate spending, the company suggested that online spend is finally set to overtake TV, which currently sucks in 38 per cent of spend - but not until 2018.
"We think television's share peaked at 39.97 per cent in 2012, estimate it at 37.7 per cent in 2015, and expect it to fall back to 34.8 per cent in 2018," suggested the report.
Meanwhile, 2018 will also be the year advertisers will spend more on mobile advertising than they do on desktops.
Of online ad spend, mobile will make up 50.2 per cent, with total global spending hitting $114bn in 2018, making it larger than all other media - except, obviously, television, which will account for $215bn.
“Growth of the global ad market is being driven by advances in technology, especially mobile and programmatic tech,” said Steve King, ZenithOptimedia’s chief executive.
“But television remains by far the most important channel for brand communication, and online video, its digital offshoot, is increasing the audiovisual share of global display advertising.”