Critical energy policy decisions rely on expert assessments of key future uncertainties. But existing modelling techniques that help form these expert assessments often ignore the existence of uncertainty. Consequently, techniques to measure these uncertainties are of increasing importance. We use one technique, expert elicitation, to assess six key uncertain parameters with 25 UK energy experts across academia, government and industry. We obtain qualitative descriptions of the uncertain parameters and a novel data set of probability distributions describing individual expert beliefs. We conduct a sensitivity analysis on weights for a linear opinion pool and show that aggregated median beliefs in 2030 are: for oil price $120/barrel (90% CI: 51, 272); for greenhouse gas price $34/tCO2e (90% CI: 5, 256) and for levelised cost of low-carbon electricity 17.1 US cents/kWh (90%CI: 8.3, 31.0). The quantitative results could inform model validation, help benchmark policy makers beliefs or provide probabilistic inputs to models.