No difference at all if they are on the same day. Go with the time that works best for you. I am not a morning person, so I would choose the afternoon but some people find the opposite and prefer doing these things first!
Yeah I ended up booking the afternoon just to guarantee I'll get there with ample time to spare (as someone who went to uni in London I remember leaving at 6/7 for a 9am and still getting stuck on the Central line due to a signal fault for 45+ minutes, getting tired of waiting, getting off and walking to uni and arriving sweaty