We live in Eltham and my son is at St Olave's. Not sure about Erith, but from New Eltham it's easy. Children of 11 do travel on public transport there without any problems and to be honest at secondary level you would expect them to be capable of this normally. By bus, they get a bus to Chislehurst war memorial, then change there. You see lots of kids changing there. My son leaves Eltham at about 7.35 to get to St Olave's by 8.40 . The other option is the train; some local kids get the train in to Hither Green and change to go out to Orpington.
I'm not sure it would be worth the long journey from Erith , unless you can find a good train route. It's just not going to be worth sending him on an arduous journey and wasting so much time, just to go to Olave's - he'll have lots of homework, and may end up with little free time in the evenings. My son is in the sixth form at Olave's and, while he's happy there, his friends who attended other schools in earlier years have been a bit disappointed and several have said that they thought the teaching was better at their previous schools - though of course, that depends entirely on your subject teachers. I think the great results will be mostly due to the strict selection criteria rather than brilliant teaching. In addition, a lot of the sixth formers are angry because the school throws students out after AS-levels if they don't get at least a B in every subject they want to take to A2. The kids have said that if the school was even pretending to care about them as individuals, it woudn't do this, but would tolerate a less-than-impeccable result if the student was settled in the school and didn't want to have to move for yr 13. Another student I know who got perfectly good A-levels said she never felt valued there, because she wasn't a straight A* student.
Your son's academic achievement will only be influenced to a certain extent by which school he goes to. The fact that he's got this far suggests that he's very clever, so if you and he decide to go somewhere closer, no doubt he'll still do well. See http://www.telegraph.co.uk/education/ed ... ching.html
and BBC Radio 4 interview with Professor Plomin this week: http://bbc.in/1ZKMW0i
Another option which friends have said they think is better, is to go to a nearby secondary school for years 7-11, then switch to Olave's for sixth form for the intensive help in getting into the university of your choice. May be worth considering. Good luck - it's not an easy decision.