Obese adults and children can be referred to lifestyle weight management services (LWMS) in which physical activity and/or dietary advice is delivered. Service providers quantify the “weight” change between an initial measurement and follow-up measurement(s), which could be 12-24 months later. A control group is usually absent. We aimed to scrutinise the various LWMS objectives for this weight change that are recommended by UK authorities. UK guidelines recommend that an adult LWMS should (A) reduce the sample mean body mass of all enrolled adults by at least 3%, and/or (B) reduce the body mass of at least 30% of adults by at least 5%. We highlight the potential for objective B to be met even if no LWMS is implemented, especially over the recommended follow-up periods of 12-24 months. This is due to unavoidable random within-participant fluctuations in weight over such periods of time. A ≥1-kg reduction in mean body mass is also to be expected, even without any LWMS. Therefore, we suggest that objectives A and B are too liberal. Obesity status in children is indicated by the body mass index (BMI) z-score. Nevertheless, another UK recommendation is for a LWMS to “maintain” or “reduce” the BMI z-score of 80% of the enrolled children. Besides there being no stated thresholds for “maintain” and “reduce”, it is inconceivable to deem a LWMS successful even if 80% of children do not alter their obesity status and even if the remaining 20% of children actually increase their obesity status. Here, we think the BMI z-score has been confused with the body mass z-score. In conclusion, measurable objectives of UK-based LWMS need to be clarified, and possibly altered to account for typical amounts of random variability in individual weight measurements over the service time period.