May I know how you define average and how well you scored in O
levels? No matter what, don't be afraid as I had friends who scored
C6 for English in O levels and still managed to do pretty well in
GP. GP will not just testing your language skills (eg. vocab,
grammar), it will also test you on your knowledge of current
affairs and how you write about your opinion on certain key
issues in the world. So you may be poor in the language, but if you
have good points to write about, you can still do well. Just try to
read the newspapers more and form some opinions and think
critically about issues.
In any case, even though passing GP is a requirement for
entering a local university, you should be able to get into the
course you want upon appeal (as long as the course is not dependent
on your English ability, eg. Engineering or Science) even if you
fail GP in A level, as long as your grades in other subjects are
good enough.