The use of molecular tools often shows that regional species diversity differs from what we know from assessments based on morphological identifications. The seaweed flora of the British Isles has been well established over a long period from foundational work published during the XVIII-XIX centuries to more recent revisions based on molecular tools. The application of these tools (primarily rbcL sequences), alongside morphological observations, to the study of the tribes Streblocladieae and Polysiphonieae in the British Isles led us to three new records of species, as well as two species that require taxonomic revision. Polysiphonia morrowii Harvey, P. delicata Díaz-Tapia and Vertebrata tripinnata (Harvey) Kuntze are recorded for the first time in the British Isles. Finding P. morrowii and P. delicata, which are considered introduced or cryptogenic in Europe, is not surprising and these new records improve our knowledge of their distribution. Vertebrata tripinnata had previously been recorded in southern Europe and collecting it on the northern coast of Ireland considerably expands its known distribution. We also found that rbcL and cox1 sequences for the morphologically divergent V. simulans (Harvey) Kuntze and P. ceramiiformis P.Crouan & H.Crouan (which has never been transferred to Vertebrata) from the British Isles were identical, and we propose to reduce the latter to a synonym of the former. Finally, we found two pseudo-cryptic species represented in specimens morphologically assigned to V. fruticulosa (Wulfen) Kuntze, and we propose the resurrection of V. martensiana (Kützing) Piñeiro-Corbeira, Maggs & Díaz-Tapia. This work further evidences the relevance of reassessing red algal species diversity using molecular tools, even in regions where floras are considered well-known.