Again, as others have mentioned (and we mean it in the kindest way!) who will allow you to wear what to get married in is so far down the line it's a moot point at this stage. First you need to meet, and make sure kissing the bloke isn't like kissing your brother if you know what I mean, amoung other things. Do you mesh as well in person as online? Can you stand this person for extended periods of time? Is h who he says he is?
You gotta jump the hurdles before winning the race, as they say
This so much. You can meet up and he's completely awesome, or you can meet up and he's a complete ars***** who creeps you out, you really don't know until you actually meet him face to face. What you wear is the least of your concerns at this point until you guys have met, and you've been to the UK to see it for yourself before committing to moving over there. It took 3 years before me and my husband got to the point where a wedding sounded like a good idea, mainly because we took that time to really get to know each other and make sure we could live together without strangling each other--the distance is a PITA, but once the distance is breached and you're married and in the UK, you're still stuck married to this person.
Your best bet is to keep things to courtrooms, chapels, or other approved avenues for marriage--I wouldn't chance trying to find someone ordained on the internet to perform the ceremony, especially when immigration is involved. If you can't find something Vegas-esque in which to get married, you might have to bite the bullet and just phone up a judge and do it that way, sans PVC--in most courthouses you have to go through metal detectors and security to get in the building anyway. Me and my husband just wore trousers and nice button-up shirts--I'm sure an appropriateness factor would be similar to what you'd wear for a job interview, or if you were going to court (since that's where you'd be getting married anyway).