Thraustochytrids isolated from hot tropical and sub-tropical waters have been well-studied for DHA and biodiesel production in the last decades. However, little research has been performed on the oils of cold-water thraustochytrids, in particular from the North Sea region. In this study, thraustochytrid strains from British waters showed high relative levels of omega-3 long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA), including docosahexaenoic acid (DHA, 22:6ω3). The relative levels of DHA (as % of total fatty acids, TFA) in the different British strains are hitherto amongst the highest recorded from any thraustochytrid screening study, with strain TL18 reaching up to 67% DHA in modified Glucose-Yeast Extract-Peptone (GYP) medium. At this screening stage, our results identify three British thraustochytrids as promising organisms for the production of DHA. Nonetheless, low level of fatty acid yields were obtained in both modified GYP and MarChiquita-Brain Heart Broth (MCBHB) media. Hence, optimizing the medium composition and culture conditions in order to increase the fatty acid yield are necessary steps for future development. In addition, under the culture conditions used and assuming a following theoretical winterization step, the qualitative properties of the oils from the British isolates could meet the full criteria for the production of biodiesel (international standards (EN 14214; ASTM D6751) after a mathematical analysis. Thus, after the production of omega-3 rich oil by thraustochytrids, the by-product oil could be potentially used in biodiesel application.