Small sewage treatment works operating under descriptive permits are widespread in the UK and may face future expectations to meet numerical secondary-treatment outcomes. Surface-flow constructed wetlands offer a low-energy upgrade pathway, but evidence for secondary-level performance remains limited. This study evaluates a full-scale secondary surface-flow wetland using cell-resolved concentration profiles, month-by-month mass balances, and areal first-order kinetics (P–k–C*) for phosphorus and ammonium. At a mean hydraulic loading rate of 0.02 m/d, the system achieved a mean final effluent BOD5 of 18 mg/L (P95 33 mg/L) and TSS of 21 mg/L (P95 31 mg/L), indicating secondary-type outcomes for carbon and solids. Total phosphorus declined along the train (influent typically 5–15 mg/L) to a mean final effluent TP of ~4 mg/L, while ortho-P decreased from 6 mg/L to ~3 mg/L. Mean areal rate constants were 11 m/yr (TP) and 12 m/yr (ortho-P). Monthly mass balances showed net retention in most months, peaking at ~1 kg/d with episodic net export (0.2 kg/d) driven mainly by dissolved P. Ammonium removal was constrained and variable (influent median ~50 mg/L; final effluent median 18 mg/L; P95 49 mg/L), with a low areal rate constant (k = 3 m/yr) and near-zero NOx throughout, indicating limited nitrification. Back-calculation from last-cell conditions showed that meeting 15 mg/L ammonium would require 1,073–4,719 sqm polishing area, versus an existing 487 sqm last-cell footprint. Overall, secondary surface-flow wetlands can deliver reliable BOD5/TSS control and typical P kinetics, but stable ammonium compliance remains the binding constraint without nitrification-supporting adaptations.