Not much would happen so I wouldn’t worry. Some European countries may well strictly enforce the 6 month passport minimum validity rule on Brits or you may well see lengthy passport delays at EU airports due to enhanced checks and Brits being asked to use the “others” rather than EEA channels. My passport is valid until May 2020 but I am renewing it within the next month to facilitate travel in the EU from October. The key is to make sure you have consumer protection for whatever you book and generally you will in the UK.