I was caught speeding through a red traffic light. Do I now face 2 offences or can they be treated as one offence?

You should not be prosecuted for both allegations. In all probability, you will receive a Fixed Penalty for the traffic light allegation and no action will be taken for the speeding. Even if you were to be prosecuted for both allegations, as these are “simultaneous offences”, you should only receive penalty points for one offence.