I apologise re the advertising, my mistake.
Firstly, I am not sure re the absence of inhibitor on the continent, all I can say is that if the water used in a system is de aerated (i.e pure water with no dissolved oxygen,[boiled water would work] and the system is sealed, then no corrosion will occur, and no inhibitor is required. It is the combination of water and air which causes the corrosion.
Re recommendations, any inhibitor that has Build Cert should work to a high standard.
The only ones that can be tested for, to see if the correct dosing has been applied, are those with a tracer element added (Mo) and they are Sentenel X100, and ours (sorry). Test kits from most good plumbers merchants.
Always check that the boiler warranty does not specify a particular inhibitor as a pre-requisite. If the boiler fails the boiler company may send a sample away for analysis, if a simple chemical test cannot be done (for Mo)
Some manufacturers do produce concentrates, in 500ml bottles that can be used at 0.5%, always read the recommended dilution rate on the bottle.