hoechststeuersatz

Hello, I've worked for a company for 10 years, of which 1 was in Hungary, 4 were in the UK and 5 were in Germany, where I live and where I ended my time with that company this March. I received a severance payment, for which the employer paid taxes corresponding to the 10% to Hungary (for my time there), but the rest was taxed entirely in Germany. When I raised this with the employer, they said they received advice that because I live in Germany, I am exempt from taxes in the UK, therefore under a 'reversion clause' Germany gets to tax the entire amount. I tried to find information on the HMRC website but all I could find is a reference that I "may be able to get part of your termination payment tax free, if you’re: not resident in the UK for tax purposes (a non-resident) for the whole of the tax year in which your employment terminates." Would my employer be correct here or do they (or I) actually have to pay taxes to the UK? Is there more information about what part of the termination payment would indeed be tax free, as I suspect the amount involved here could exceed the threshold? Thanks!