The reason for the arrest is that this person was banned from entering the UK but somehow managed to evade customs and immigration on entry. This in itself is illegal. There was obviously an initial reason for banning the person which would have to be extreme hate speech given the UK's tolerant attitude to Islam.
Syrian group says Nusra abducts its leader, in blow to U.S. plan (Reuters)
from the article: Islamic Movement leader in Israel arrested in London