Why does this happen to me in nightclubs- is it because i'm white?

It is possible that they could be attracted to you more so than the darker-skinned women simply because you look different and stand out more. Or, it could be a cultural misunderstanding. You say you don't dress provocatively, and that most likely is in fact true- by Western standards. But in many of the Eastern cultures to which the men above belong, anything short of being covered from head to toe in baggy, loose-fitting garb is considered provocative, and that is why they are targeting you. They simply might have never seen much of Western attire and culture and so have never learned to deal with it. There is no excuse for their actions, however, and you have the right to be treated with respect. I don't know much about London, but you could try looking for clubs with mostly European or American clientele. Their beliefs should be more compatible with yours. If your current friends don't like this idea, then you could probably find some new buddies in the more Westernized clubs. It's not racist; it's about standing up for yourself and your rights.