By the sound of it you are more then capable to race at a national, I currently have a F5 grade, and I have applied for 4 nationals this year! If you intend of racing you can find an entry form here
http://www.onetenthoffroad.co.uk/. Regarding the F grades the higher your grade the more likely you are of getting a placement at the event, however racers from all grades do race at these events!
Hope this helps a little!
Kev