Goran , I think we all suffer GPL nerves to a lesser or greater extent , I know I do for sure escp during qually .
Race craft comes with experience , here's my take on racing, for what its worth .
For me the nervous bit settles down during the race after a lap or two , I try to find a rythm , a speed that I'm happy driving at regardless of who I'm racing and stick to it .
Once you have settled down into the race you can try to push a bit harder to get the guy in front , but not too hard because thats when the mistakes happen .
Most important is to look for him to make an error in braking or running wide or slow corner exit , that will give you the overtake chance.
Of coarse if he'se very good ( and a lot of UKGPL guys are exceptional ) they may not present you with the oppertunity , then if your faster you will be able to make it happen , by outbraking , getting alongside and ahead into a corner or making a faster corner exit speed . Easier said than done.
Best advice is to settle yourself down and "enjoy" your racing , the experience will come with practise