"The timescales do vary from emergency to whenever possible, but there is a within one, two or three hour option in between." So by my reckoning there's still a gap. It's between immediate response and "within one ... hour". Let's take a concrete example. I'm an asthmatic and no stranger to ambulances, sometimes replete with full blues and twos onto the way to A&E. There have been times I've needed them here within 8 minutes and there's times I've needed them soon but not so urgently that I'd want someone running blues and twos on their way to me, but also not as far away as an hour. I'm sure that's not the only example of a situation where (when there's someone competent on-scene to grade the response) there's a call for a grade "on the hurry up, but not so much as to routinely call for blues and twos".