I think that’s pretty standard for a 3 channel junction box. If you look at the wiring diagrams drayton provide for their 3 channel system it looks similarly complicated.
It looks like in @rr4444’s case if the heating channel from the LP241 was ever off, it would kill the 2 BDR91s, as presumably it is switching the 230v power line to those controllers. I don’t know if that means the wiring in the 3 channel junction box is then non-standard or not, but it looks like a non-trivial re-wire.
Essentially I believe the control pairs from the BDR91s need wiring into the 2 heating channels of the new backplate, and the 230v power needs disconnecting from both. The 230v power to the LP241 needs wiring to the power of the new backplate, as does the hot water control pair. All other wiring connected to the LP241 needs to be removed. That should leave you with mains, HC1, HC2 and HW on the new backplate.
If this were my heating setup, I’d be tempted to re-wire the whole thing from scratch, using the drayton installation guide for reference. This looks a little easier in your case as lots of the wires have been helpfully labelled.