Registration Certificate?! I do not know what that is, but I have proof of having been registered as a PhD student at UCL for the whole period. (which the HO acknowledged as being adequate to prove I was a student but for proving employment they need payslips apparently.)

I was a PhD student at UCL from 2010-2016. The HO acknowledged my worker status, but only for a limited period, namely until I was receiving funding from UCL. The funding stops after three years and the last two years I was no longer receiving money from UCL but of course I carried on my work as a ...