Hi, I remember having problems with this when I applied, as because I was also starting a new job, I needed my degree ceritificates to be in two places at once and had no time to get certified copies. As I was going over for an open day, I simply rang up the university and asked them if I could bring the originals with me then. They had no problem with that - and also said that even if I hadn't been attending an open day, I could have brought them to my interview instead.
Basically, my point is that if you're having real trouble getting certificates/certified copies to them within a given deadline, you could always give them a ring and offer to bring them in personally at a later date.