I assume that when you state that you've achieved Key Skills Level 2 in 'maths', you actually mean in 'Application of Number'. [As a former maths teacher, there's no way that you'll get me to agree that 'application of number' is synonymous with 'mathematics' ;-) ]
Key skills Level 2 is equivalent to a GCSE pass in the A* to C bracket, so you can certainly say that you've increased your total number of GCSE (A*-C)-type passes. However, you need to be aware that not all employers have heard of Key Skills so, if you don't re-take GCSE Maths, you'll need to be prepared to explain that your qualification is equivalent to a GCSE pass at grade C or higher.
Source (which is a Word document, not a web page):
http://www.keyskillssupport.net/files/teaching learning/studentguidance/studentpub/introducin g_keyskills.doc
Chris