At the majority of NATS airports the observations are done by ATSA's using the SAMOS (semi automated met obsevation system) equipment. Most units now will have ATSA's who are met trainers/examiners and will include met observation as part of the overall training.
Two NATS units - Luton & Southampton - still provide METARs on the basis of 'manual' observations i.e. without SAMOS. At these locales, all ATC staff, ATCOs and ATSAs, are Met. observers having been required to undertake the two-week 'full' Met Office training course.
Nationally, it doesn't matter whether you're an ATCO, a FISO, an ATSA or a firefighter, if you've attended and passed the Met. observer course and your unit's Met. equipment is 'approved' by the Met. Office, then you can issue METARs. If not, your observations are classed as "unofficial" and must be prefixed as such when being passed by telephone or R/T.
