The ICSP on the Uno, has the same function as the ICSP on the REV7/8 - they both receive the programming information for the processor.
There's stuff here about that: https://www.arduino.cc/en/Tutorial/ArduinoISP
So you'd use a Uno as the source of the programmer, and the REV7 or REV8 as the target to be programmed. This is not as you've described, so I'm hoping that this has helped shift your orientation.
All the OpenTRV boards run at 3.3V, so supplying them with 5V is likely to do bad things to the RFM23B which is not 5V tolerant.
You can happily power the REV8 from it's own power supply. You'll still need ground connected.
There's no guarantee that the Arduino 3.3V will supply sufficient current for the board to be programmed successfully - but it might...