Always try 999 even when there is no signal or credit on your phone. Mobile operators in the UK have an Emergency roaming agreement whereas a 999 or 112 call to the Emergency Services will always be routed through any available mobile network.
Person struggling in the water? 999Coastguard.
Person gone missing on the coast? 999Coastguard.
Person injured on a coast path or beach? 999Coastguard.
Person or a pet fallen over a cliff or in the water? 999Coastguard.
Any emergency on the coast, think ☎️999#Coastguard.