我係英國讀完 a-level (= 香港的中7程度) 點樣可以報香港d大學?

yeah. But you have to be quick. You need to get all your certificate ready and you need to print a reference letter from the web for your tutor to write.
The deadline for HKUST is 28th Dec;
HKU is 31st Jan
Poly doesn't start until 19th Dec
and CUHK is about Feb.
I have done the applciation for HKU, HKUST and CUHK. You need to spend a lot of time working on the personal statement and I personally find that the courses in Hong Kong is really limited, unlike those in HK.
I think you need to check the style coz it'll be completely different to those in UK. You might not be able to cope with the uni in HK
I recommend you to apply online because the fee for application is cheaper. You have HKID card therefore you will apply for Non-jupas (direct admission for CUHK) and you are under Local students. Don't try to apply for international students or your school fee will be really expensive!

What is your gcse result?
are there lots of A* in GCSE or not?
how many A in A-level?

yes, no problem apply through non-jupas ie. directly through uni
have a look at their website