People have been reading the current so called lockdown regulations in the most restrictive way possible, it is clear that there is nothing in the regulations to stop you flying the aircraft to a place of maintenance and for maintenance work to be carried out.
Pure Leisure flying is not appropriate but it is clear from the government guidance that the industry needs to be in a position to re-start as soon as practicable and as long as appropriate social distancing measures are observed preparing for the re-start is a wise and economically essential course of action.