Considering youre retaking GCSE's, not to sound as if im belittling you, but you probably do need the two years.
The reason for this the difficulty is way (WAY) harder than GCSE, and its hard to get a good grade even if you spend the full two years, a few people end up resitting a year and taking three. Sure if you're a star and got all A* in GCSE you might well be able to, but judging from your post it doesnt seem so.
Yet again being an optimist and all is all good but I'm just being realistic, dont put yourself at a disadvantage purely because you want to go Uni quicker.
If it makes you feel any better, alot of people are in their 20+ at their first year in uni and no-one cares or realize it. Its completely fine to be 20 when you go uni don't worry about it.