As i thought its the New Hovis bread advert. it cost 15 million to make. here you go.

Hovis launches epic “longest ever” TV ad
Breadmaker Hovis has launched an impressive new ad to celebrate its birthday, charting a tumultuous history of Britain.
The epic advert has been created by the bread maker, moving on from its traditional “boy on the bike” adverts. It is the longest ever television commercial, standing at 122-seconds. It cost £15m.
Beautifully showing Britain’s highs and lows through time since Hovis was founded, including the first and second world wars, the Queen’s coronation, England’s world cup win, the miners’ strike and the turn of the new millennium, viewers and critics alike have called the ad “tremendous”.
http://www.boxzone.net/uk/hovis-launches-epic-longest-ever-tv-ad.bz

I'm not sure how you would define the difference between 'A party Political broadcast/Infomercial' and a purely commercial advert - but it does feel as is there is a difference. I guess this might be down to definitions.

There was a Carling Black Label advert shown in the 80s which took up an entire commercial break.

Here's a Youtube video of it, although I'm not sure this version is complete because I'm sure the original was longer than 1:22

The longest one I've ever seen was for the Sydney County Council (the electricity supplier at the time). It featured the ride of the valkyries as the theme music. It ran for about 90 - 120 seconds I believe.