bro the reason is that,still,so many muslims [especially pakistani mirpuris!] force their daughters --usually via emotional blackmail -- to marry a cousin from back home with the sole purpose being to get the spouse an easy visa to the UK. It is assumed that at 21 girls will be less prone to such pressure. It happens to guys too but much less. Of course it stems from the ruling that a father [or grandfather etc. or wali] may marry a virgin off to someone even without her consent in the Hanafi school [and not sure of the other 3 Sunni schools but they're probably more strict!].
i think 18 was fine. it is probably another way to curb or at least delay the immigration. 21 for me is too high because there are genuine marriages where the spouse abroad is under this age. this will just cause them unnecessary headache. some may see it as a violation of human rights.
this will affect quite a few asian in the UK. i knew about the law but didn't expect it to happen so soon after the announcement. http://news.bbc.co.uk/1/hi/uk_politics/7708305.stm
