Loads of people do it
With or without GCSEs etc. There's a Yahoo group called something like HE Exams (can find the link for you if you want) which is really helpful, and full of great success stories.
But that's for the future
Having had one dd start school at 11, and one be HE'd until 13, certainly the first 2 or 3 years are pretty straightforward to do at home. You can follow the syllabus if you like - loads of resources available - but you might prefer not to be restricted to it
For humanities there's not really any building on knowledge, it's mostly separate topics, so you could do as you pleased for them. My dd2 did no formal history ever, and is now doing it at school for GCSE no problem
I was prepared to get dd2 through GCSEs at home - there is outside help available for anything you're not so confident of (for me, languages - we were already going to an HE Spanish group - and Eng Lit - there are a few HEors offering distance learning or group tuition for English courses). The biggest obstacle in my mind was the logistical aspect - finding local/not too expensive exam centres for everything she wanted to do. People do do it, but I was quite relieved when she went off to y9!
The previous years had been great though - loads of ebay-acquired science kit and experiments at home, interesting lectures in interesting places, the Spanish group, Latin, and the pleasure of having an older child doing more challenging work and having interesting conversations.
Socially I found I had to look a bit harder with an older child than when they were young, but this very much depends on your area - there are huge numbers of older kids around in some places.
My dd2 flatly refused to try for the grammar school, and there were no other secondaries that I was happy with her going to at 11. The good thing about going a bit later was that she was confident to travel a bit further, and so our potential choices increased greatly. She was also very clear on what she wanted from a school, and so when we started looking in June when she was about to turn 13, there were about 17 schools to choose from. Because she wanted to do Spanish gcse and have the option if 3 sciences, that was whittled down to 4. 2 had long waiting lists, one had a place but she wasn't that keen, and one she liked and she was put top of the waiting list - a place came up a couple of weeks before school started in September.
So even if you do end up HEing next year, you don't have to go into it thinking that that's it for the next 5 years! I think going a bit later when they're a bit more grown-up can be really good for them, and have seen several HE kids I know go to school at 12-14 and be very successful.