Pre-charge detention: a simple question

In the UK police can detain, without charge a suspect in a terrorism case for 28 days. In the USA the police can only do so for 7 days. Why do we need four times the pre-charge detention period when no-one will sensibly argue that we have four times the threat of terrorism?

I’ve put the question to Opposition spokesmen as well as a Home Office minister and never got an answer. With all the other new law enforcement powers proposed (post-charge questioning, admissibility of intercept evidence in court and, most importantly, extra resources so the police can investigate quickly) shouldn’t we be thinking about reducing detention without trial back down to 7 days, not increasing it further?