Can I have an open vented pumped CH system and gravity HW with a
condensing boiler?

No you can't.

Note that this requirement isn't about condensing boilers. There is already
a requirement to upgrade to fully pumped and controlled hot water cylinder
heating when replacing the boiler and has been for some years. So even if
you replace right now with a traditional boiler, it would be against
building regulations to keep the gravity hot water primary circulation.

Christian.