Each of the countries making up Britain has a slightly different system for appointing arbiters. Potential candidates are advised to look on the respective website (see links) for contact details.
However, each country has a similar system at present of sitting an exam and requiring norms from performance as an arbiter at events.
FIDE also has a similar system. The FIDE titles are FIDE Arbiter (FA) and International Arbiter (IA), the latter being the more senior title.
There is also the NA Title which is necessary to be an arbiter at a FIDE rated event. Countries decide the minimum requirements to be put forward for this title