Okay. So the mixed flow being less than the cold alone remains a mystery. But basically the mixer tap works and were the issue flow rates alone, I'd suggest the customer ought to be happy. 4.2 l/m hot is borderline for a kitchen tap, but if the customer wants a mixer, that may be about right.
An installation in breach of the Water Regulations (unless the plumbing firm is a Watersafe Approved contractor) leaves the customer liable to (unlikely) prosecution. I would recommend the OP contact the local Water Board ("Water Undertaker") and discuss the matter with the Water Regulations department. If check valves are not fitted, that is probably a breach of the Water Regs (nothing in the Blanco link supplied suggests check valves not required).
But fitting check valves here would probably ruin the flow rate on the hot side so the company that supplied and installed the tap should have initiated a discussion in which the company states the tap installation won't flow properly (once the check valves are fitted) so does the customer want to have a hot water pump fitted/pipework upgrades or would he or she not prefer a more suitable tap instead? I'd have that conversation - which is probably why I'll never be the cheapest.
I would not have wanted to fit that specific tap to a mixed-pressure system and would not expect the customer to know anything about plumbing hence I would not have recommended it. I'd have suggested a biflow tap (no check valves required) and, as for the time to get hot water, how was the old tap and what does the customer want to to ensure the new tap is no worse? At a guess, I'd suggest the firm may have good intentions but possibly lacks a qualified plumber - hence a lack of technical understanding.
For what it's worth, the hot washbasin tap in my own home is a 1/2" hot only tap. I also have a vented cylinder. The bathroom is on the first floor, the cistern on the loft floor above the first floor and the house has low ceilings. I probably have 1.5m head. I can't remember the exact flow, but, from memory, I'd suggest it's nearer 9 than 4.2 litres per minute (possibly it's as much as 12). I have hot water in under 15 seconds. Yes, the pipework was designed and installed to the nth degree, but what really helps is the tap mechanism is to BS1010 and specifically designed for 0.1 Bar use. The continental firms don't really get British systems as the UK refused to accept mains pressure hot water was safe until the foreigners had used it for 50 years, so our older-design mechanisms with washers really do work best with lower pressures.
Try giving PeglerYorkshire a ring and seeing what the flow rates are for their 0.1 Bar kitchen mixers?
The problem with changing the isolators is that there isn't a huge flow rate to begin with, so such a restriction as a 10mm bore valve won't be causing very much restriction: the restriction, it would seem, is the tap itself, but it may help - slightly.