We don't mind too much when it starts but the sort of spectators who don't plan their days (these days, this includes my dad) around the race du jour do mind. Hence Claire's comment.
Time of broadcast is now more important to me than literal race start, since there's only one race per year (the Chinese) that I can watch on an all-live channel, and for that race I'm rarely around to watch it live anyway (because the reason I can see it on an all-live channel is because it's in the Silverstone Snoozebox, and the reason I'm there is for another race entirely... ). There have been several times that I have nearly been spoilered, including once by Mum (who knows full well that Dad and I avoid spoilers of any kind if we have to watch highlights, but thought it was impossible for me to have missed hearing about that one item). In 2012, going shopping after the race was due to start meant a risk of being spoilered while shopping, but that's not such an issue now.
(I am in Derbyshire, which is in the north Midlands, the south North of England, or a proto-border-of-Scottish town depending on which news report you believe).