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Unlikely. A low loss header is a small tank that takes the flow and return on the boiler, then this tank has further outputs with external pump(s) which does all the heavy lifting to the rads. If the most distant rads were not heating even with the boilers own pump on full, then an LLH and further pump would sort this.

Your issue is the opposite, too much pressure on high flow resistance. The boiler has 6 flow settings. O is auto, 1 to 5 are fixed, low to high. Your current setting can be read in the diagnostics (p27) as diagnostic code D14. If its a fixed rate, 1-5, you could try auto, 0, or a lower fixed rate. A summary of diagnostic codes is on page 46. You might want to leave this adjustment to the installer, they also warn on p27 about entering diagnostic mode as a 'competent person', which probably excludes Jedward's handyman.

Fair play. There should be a handyman (with a quiff) code. Auto should do it, 5 was a fixed top speed. There's a caveat in the manual (10.4, page 35), not to change the pressure adjustment screw when in Auto, which makes sense. It also has a chart which basically says you should increase the pressure screw setting ( when default is 1 to 5) if distant rads don't heat, or reduce it if you can hear valve noise, but also states not to touch this screw if auto, = 0, is the factory default. It seems a little confusing and contradictory, but as long as you leave that pressure screw/bypass valve at its current setting, I can't see the harm in selecting auto, then test for all rads/ two or less rads. It should spool up to high enough to circulate all rads, and trim back as TRVs drop from the loop.