If you head into wild places, whether on the sea or land, then you really ought to do this. It takes less than 45 seconds.
In an emergency situation, where you might not have a strong enough mobile phone signal for a voice call but a text might get through, you can use that text to alert the emergency services.
If your phone is registered.
Send a text to 999 with the word REGISTER. Read the reply. If you're happy, follow the instructions and reply with the word YES.
If you're in any doubt, I suggest you read this. Honestly, why would anyone in the UK not do this?