The reason I am losing 50% of the boiler flow is because the header gate valve (by-pass) is doing its job and reducing the harmful 30°C delta-T from the cold radiators down to 15°C across the boilers.
The Celsius 25 installation instructions on P11 has a diagram showing an additional system pump, which requires a permanently open by-pass (LLH, short circuit).
http://www.keston.co.uk/downloads/manuals/Celsius-25-manual.pdf
The 2 boiler reverse-return arrangement, with the ABV, shown on the link below, is intended for direct connection to the distribution (secondary) system.
http://www.keston.co.uk/downloads/support_info/multiple/2xcelsius-v2.pdf
This design assumes that "the system resistance is within the spare capacity of the boiler pumps. Otherwise consideration should be given to the use of an additional System pump."
So, the boiler pumps can be used for the secondary distribution, if capable, and an ABV is required. If an additional system pump is required, you then need an open by-pass/LLH.
The LLH is preferable since otherwise the secondary flow will vary enormously depending on whether one or two boilers/pumps are running.
The single boiler header diagram shows 1 ½” pipework was recommended for one Celsius 25.
http://www.keston.co.uk/downloads/support_info/multiple/1xblr-h.pdf
I still don't think the undersized LLH pipework is a major problem. There is a CIBSE guide which I believe recommends the pipework for a LLH should be selected so that the pressure differential at the secondary F&R connections is no more than 40 Pa, i.e., negligible pressure loss. It would be easy to look up the pressure loss per m for the LLH pipe to see what the pressure difference is at the two primary flow rates.
The differential pressure (across F&R) could cause nuisance flow in a secondary circuit when you don't want it and the pump was off. Since both the CH and DHW circuits on this installation have zone valves, it will not cause a problem.