The headline "UK growth shrinks to worst level since 1990" is not incorrect, but would be better written as "UK economy shrinks at fastest rate since 1990". The first paragaph reads:
Britain's economy shrank during the third quarter of the year for the first time since 1992 and endured the worst single quarter since 1990, it was revealed today.
One should be careful here. GDP in Q3 2008 was probably something like 40-50% higher than it was in in any quarter of 1990. So when saying it was the 'worst' quarter, the author means the rate of change was the worst since then. Although this is how these things are written about, they shouldn't really. If you earned £11,000 in 2004, £10,000 in 2005, £15,000 in 2006, £40,000 in 2007, and then £39,000 in 2008, you probably wouldn't declare 2008 your 'worst year since 2005', at least without some further explanation.
I think the Guardian covered it better.