Public perception of regulated sexual behavior is a crucial factor in considering policy, particularly when UK sexual consent laws could criminalize the many young people who are increasingly engaging in underage sex. In contrast, other countries have progressively implemented consent age-spans to respect sexual autonomy while simultaneously protecting young people from sexual exploitation. This study investigated UK perceptions of consensual underage sex between similar-aged adolescents, with predominant focus on age-spans, as an adaptive alternative to dropping consent age. An online experimental survey of members of the UK public (n = 336) manipulated age-gaps, perpetrator gender, and respondent gender. Supporting the hypothesis, results showed more support (i.e., reduced condemnation of the situation, perpetrator, and victim) was given for sex with age gaps of up to 2 years than those larger. Gender differences were present only for age gaps of 3 and 4 years, where harsher situation and perpetrator judgments were elicited when the perpetrator was male. To our knowledge, this is the first UK-based study investigating perceptions of similar-aged consensual underage sex, and therefore forms a baseline for future research. Results are discussed in terms of acceptability of behavior and present important evidence for policymakers to consider reviewing UK consent laws in line with international legislation.