Not many people around here own Naim kit, so don't be surprised if you don't get a satisfactory reply.
Onboard headphone outputs can be quite poor, but there are also excellent ones out there. I recall for example that the headphone out of my NAD C352 amplifier was on par with the Musical Fidelity X-Can v3 I also had at the time.
Since I haven't heard the UnitiQute, I can't say for sure, but I expect its headphone out to at least surpass £50 amps and maybe more. Just give it an honest listen if you can.
Better sound quality is not the only reason dedicated headphone amps are produced. Other reasons to make or buy them are added functionality, smaller size, or minimalist ideas. What I'm trying to say is: it may well be that the headphone output of your UnitiQute is great. Don't let the amp hype confuse you.