We use a set of biomarkers to measure inequality of opportunity (IOp) in health in the UK. Applying a direct ex ante IOp approach, we find that inequalities in health attributed to circumstances account for a non-trivial part of the total health variation. For example, observed circumstances account for 20% of the total inequalities in our composite measure of multi-system health risk, allostatic load. Shapley decompositions show that apart from age and gender, education and childhood socioeconomic status are sources of IOp. We propose an extension to the decomposition of ex ante IOp to complement the mean-based approach, analysing the contribution of circumstances across the quantiles of the biomarker distributions. This shows that, for most of the biomarkers, the percentage contribution of socioeconomic circumstances, relative to differences attributable to age and gender, increases towards the right tail of the biomarker distribution, where health risks are more pronounced.