In this work, an approach toward reaching new insights into the mechanisms of superconductivity and the effects of ion irradiation in the iron-based compounds is presented. We investigated the critical temperature and penetration depth of high-quality Ba1-xKxFe2As2 single crystals by a planar waveguide resonator technique, in a cavity perturbation approach. Measurements were repeated for pristine samples and for samples irradiated with 250-MeV Au-ions at different fluences. The experimental method and data analysis are described, showing that this technique is reliable for the study of small crystals and, since the measurement technique is non-destructive and does not alter the crystals, the very same samples can be measured before and after irradiation, making the analysis of the effects of additional defects more reliable. The absolute values of the penetration depth are accessible by the experiment, showing a fairly good agreement with earlier literature data (λL,ab(5K)=220 nm for the pristine sample). The experimental λL(T) is also compared to calculations based on the three-band Eliashberg equations within the s+- wave model giving a remarkable agreement. This overall consistency validates the model itself, thus allowing us to estimate parameters that are missing in literature, such as the plasma frequency for each band. The calculations are also able to explain in a consistent way the effects of disorder both on Tc and on λ(T), by suitably accounting for the impurity scattering due to the defects created by irradiation. In particular, a change of curvature in the low-temperature λ(T) curves for highly irradiated crystals is fairly well reproduced.