In all fairness I don't expect any noticeable change at UK airports. The UK is not a part of the Schengen zone, so there is already passport control on entry even from the EU, Brexit will not change any of this. Customs is self-declaration by chosing the appropriate channel, and EU/non-EU traffic is already mixed at the baggage halls, so again no change (only the blue corridor sign needs to be changed to green). The only possible hickup could be if the UK would decide to no longer to accept EU ID cards for travel (and many people traveling in EU do not have valid passports), but that likely would have UK citizens end up in the 'others' queue at all EU airports...
On departure there is neither customs nor passport control out of the UK, so again no impact whatsoever.
The whole surrunding hype feels a bit like Y2K with predictions of aircraft faling out of the sky at midnight...