I don't think the fork is OEM because the blade end treatment is not the same as the stays. The stays have domed ends where they transition to the dropouts, while the fork ends are bevelled. Frame characteristic look like a higher end product from a mass manufacturer. The Portacatena dropouts date it circa 1978-1983. It's probably towards the earlier end of the range given the exposed nut brake bridge and apparent lack of braze-ons. It does look British to me, with the wrap-over stays and top tube, cable stops.