UK SAR is run by a Rescue Coordination Centre (RCC). A continuous listening watch is kept on 121.5 MHz, and an Alerting Service is provided to all aircraft known to ATS in the UK FIRs, overdue action normally begins if you do not arrive within 30 minutes of your ETA (so filing a flight plan gives you a SAR watch, strongly advised if flying more than 10 nm from the coast or over remote terrain).
The ground-to-air visual signal codes (laid out at least 2–3 m long) are pure recall:
V: Require assistance.
X: Require medical assistance.
→: Proceeding in this direction.
Y: Yes / affirmative.
N: No / negative.
An aircraft acknowledges a ground signal by rocking its wings (by day) or flashing its landing lights on and off twice (by night). No acknowledgement means not understood.