The honest answer is that "best ROI" depends heavily on what you sell, who buys it and how long they take to decide. That said, some patterns hold true for most UK small businesses.
Email to an existing list almost always comes out on top once you actually measure it. The cost is a few quid a month for a mailing platform and some time writing, and the audience has already said yes to hearing from you. Repeat business from past customers is far cheaper than winning new ones, so anyone not collecting emails at the point of sale or enquiry is leaving money on the table. Just keep it compliant with UK GDPR and PECR: clear opt-in, easy unsubscribe.
Google Business Profile and local SEO are the quiet winners for anything with a geographic angle, from trades to accountants to cafes. It costs nothing beyond the effort of keeping the listing accurate, gathering reviews and answering questions. For a lot of service businesses this single free listing outperforms every paid channel combined.
Paid ads (Google Ads, Meta) can deliver strong returns, but only when the numbers are worked out first. Know the lifetime value of a customer and the conversion rate of the landing page before spending. Without that, it is easy to burn through a monthly budget with nothing to show for it. Google Search ads tend to suit high-intent purchases; Meta works better for impulse or visual products.
Organic social is the one most people overestimate. It builds familiarity and can feed the other channels, but tracking it back to revenue is difficult, and the algorithms throttle reach unless you pay. Treat it as a supporting act rather than the headliner.
Whatever the mix, put basic tracking in place: UTM tags on links, conversion goals in analytics, and a simple "how did you hear about us?" question on enquiry forms. That last one is low-tech but often more revealing than any dashboard.