Have you seen the SSCP scoring criteria? It is online, if you google it there is a link. It shows how they award marks for different grades vs other experience.
I have been told that the NERC GW4+ DTP usually has 1200 applicants a year, so I would imagine Imperial is equally, if not more competitive.
Speak to the supervisor of the project you want to apply for, they can tell you about the DTP and if you have a chance.