For a two-dimensional hard-disk boson system we propose an analytical London-type expression for the ground-state energy as a function of density ranging from zero to close packing. Such an equation interpolates between the leading term of the exact low-density series expansion and the behavior expected at close-packing based on uncertainty-principle arguments and determined by the correct value of the residue which is a dimensionless constant whose value depends on the space dimension as well as on the kind of close packing configuration involved. As for any reasonably accurate equation of state, variational Monte Carlo calculations for both fluid and crystalline energy branches turn out to be close upper bounds to the resulting so-called London-Hubbard energy which includes pair-correlation defects.