OK the perceived wisdom is that winter is for low intensity training, IIRC basically in order to prevent burn-out/waste motivation when high intensity is not needed?

But if you have limited time and/or CBA with longer night-rides, is there any downside to doing say one weekly interval session (45m) instead of a 2 hour ride? (My theory being that the overall workload is similar)

Absolutely no downside at all.

Intervals are still very applicable in winter, however they tend to be longer in duration and therefore not as intense.

Winter is not all about long slow miles.