The problem with these gangs is that it isn't your teeth you'll lose if you put up a half hearted fight, or doubt yourself, so if you think for one second that they are more than capable of killing you, then you have to hand your stuff over.
Tottenham Hale is a really crappy area, but you can get mugged anywhere. I was first mugged when I attended college (in Debden), and it really shook me up and ended up with me too scared to go in and I subsequently fell back on my studies. It passed a couple of years, and I sort of got over it, mainly by taking up Rugby and sort of toughening up with age, and some advice from my dad, who's a Police Officer. I was then approached by a gang of about 6 lads at Loughton Station, but none of them seemed older than 16, the lairiest one tried it on, reaching in his pocket and implying he had a knife, encroaching my personal space and threatening me. I knew if I stood up to him, the others would lose their courage (if you can call it that) so I told him to 'go away' (but using the f word and probably the little c word) and he pushed me, so i just grabbed him round the throat and told him if he tries anything else i would push my thumbs so hard that I'd embed his larynx in his spinal cord (probably not possible, but sounded crazy and scary enough to work). I then let him go and he was nursing his neck and his mates all ran off.
Anyway, my dad used to be part of the proactive robbery squad, and he told me that the gangs operating on the tube aren't really that hard, and they just want to get it over and done with as easily as possible, if you think you can do some damage to the leader, the others will back off, but you have to try and act scary/unhinged to be able to protect yourself. The flipside to this though, is there is always the chance that you get a gang who will hurt you badly even if you give up your possessions, so to the original poster, you are lucky you didn't get hurt after.
Yeah, the backpack advice is sound advice, try not to watch tv or films on your iPod or iPhone when your on the tube, and try to look a bit insane if you find yourself alone on the tube with a gang.then wait till the doors are about to close, and get off before they can. (only really works on the district line...)