No need to clear security (flights from US/CANADA don't need to reclear security). No need to go through passport check as UK is not part of Schengen even now. Thus you'll get off plane and be able to head straight to connecting flight. Coming back depends on agreement that is reached between UK and EU regarding security, as UK will no longer be in EU, it could be subject to needing to reclear security. You however wont need to worry about passport control, as even now flights from UK land at international side as UK is not in Schengen area.