The pedestal structure, ELM losses and linear MHD stability are analysed in a series of JET-ILW H and D type I ELMy H-mode plasmas. The pedestal pressure (p_PED) is typically higher in D than in H at the same input power, with the difference mainly due to lower density in H than in D. At the same input power, the pedestal electron pressure gradient is typically lower in H than in D at similar pedestal pressure width. The neutral penetration model is in contradiction with the narrower (or similar) pedestal density width in H than in D. The ELM energy losses are found to be dominated by density loss both in H and D. At low ELM frequency (f_ELM), ELM particle losses increase with f_ELM, in correlation with decreasing pedestal top density (n_e,PED). Thus, the observed higher f_ELM in H than in D at same input power possibly contributes to the lower n_e,PED in H. However, ELM particle losses saturate at higher f_ELM, both in H and in D, implying that other mechanisms such as pedestal transport may also play a role in the lower n_e,PED in H. The direct isotope effect on pedestal stability, calculated with the ideal MHD HELENA/ELITE codes, is found to be small and thus cannot explain the difference in p_PED between H and D. Interpretative EDGE2D/EIRENE simulations with simultaneous upstream and outer divertor target profile constraints indicate higher separatrix temperature in H than in D for a pair of discharges at similar stored energy (which required higher input power in H than in D). This in turn translates into significant destabilisation of Peeling-Ballooning modes in H compared to D, which is consistent with type I ELMs being triggered at lower pedestal pressure in H.