Considering some of them seem to patrol tube stations at rush hour with MP5 sub machine guns they aren't exactly warm and cuddly friendly police either.
Fact is in a busy station on a morning even if you knew someone was a terrorist and could see him you couldn't actually shoot him without gunning down about 30 civilians between you and the bad guy. An MP5's on display is a show of force against the populace not a reassuring sign of protection.