Hey,
shit man, I know that feeling very well. I feel way more comfortable and talkative online than I do in person. I met up with an online friend a while ago and I was brickin it, because I'd been so open and totally myself over email and msn, but I knew I'd get tongue tied and introverted and depressed and I'd freeze up in person, and I was terrified that we'd have those "moments" where it all gets weird with awkard silences etc....luckily it turned out really well in the end, as it often does....
but anyway, my advice, if you'd like it, is to just be yourself, which includes all your social fears, cause that's a part of you too. I think you should just be yourself, because you're an awesome person, shyness included. I think everybody's got problems...everybody's got crap that they'd rather others not see....and everybody's insecure that others will reject them upon discovering this crap....including this bloke, I'm sure he's just as imperfect as everyone else.
The fact that he may know more about London already than you do shouldn't be a source of shame....not in my books anyway....I think it's charming that you don't know much about london [although you probably know more than you think!], cause it shows that you must be passionate about other things.
if I didn't have social anxiety, I'd say that meeting this guy before uni starts could be a great idea...you could make a new friend! What's this guy like? Do you think you could make good firends? Cause if you don't even like him then sod it! But if you do, then I reckon you should give it a bash! Taking risks is good for the soul!
also, he's probably not, but you'll make sure he's not a stalker or anything won't you?!!....don't give out too many personal details and meet him in a public place etc.
but anyway, my last bit of advice would be, even if it goes terribly, I still reckon taking that risk and giving it a bash will be better for you than avoiding it and staying at home. So go for it!
and good luck at uni btw! you'll love it!