Try navigating under the London CTA, say from Biggin Hill to Barton.
Agreed, not easy - but the sort of route I would have flown 30 years ago BEFORE the days of GPS (and aids and airspace on that route have not changed much since then), again, a
lot easier WITH GPS, but certainly not "almost impossible" and that still leaves an awful lot of routes that are actually easy to fly, plus if you fly CAS, even just a limited amount - which ATM is still possible without a GPS, it actually becomes an easy route!
As I said, it was the generalisation that IFR Nav is "almost impossible" without GPS that I object to, as in the example you give it makes it MUCH easier to have it, but fly say Goodwood to Sywell Via CPT and "almost impossible" becomes "piece of p!*s", and there are plenty of places that you can still fly that this applies to, so "almost impossible" for certain applications/routes, yes I agree, "almost impossible" in general - no.
Edited to add - even with GPS, which way would you get from S of the MAN TMA into Barton?