Metal Police Whistle with Lanyard: Super Loud Emergency Distress or Sports Whistle
Small, easy to carry, extra loud, high decibel police whistle with lanyard

If you don't use it for sports, this is one of those things that you hope you never need.
But it's small enough and cheap enough to get anyway.
It's an extra loud whistle that you can carry with you at all times and use in the (hopefully unlikely) event that you need to summon help or to referee a match.
You've doubtless seen TV shows where people call for help but you also know that, sooner or later, your voice would go if you had to yell for help for any length of time. Plus there are lots of places where a voice wouldn't carry far enough for people to hear you.
But so long as you can puff into this whistle, you'll be able to attract attention from anyone within ear shot - which, because it's a whistle, will probably be further than your voice would carry.
If you know any morse code, you could even send a message. The easiest message to remember would be the international SOS distress signal - a continuous sequence of three dots (short blasts), three dashes (long blasts) and three dots (short blasts again). It's instantly recognisable and will attract the attention of anyone who can hear it. They'll be able to use it to locate you and come to your rescue.
I've not got anything to measure the decibel level this gives out (the manufacturer claims 120 decibels) but can definitely attest to it being loud!
It's made from steel with a yellow lanyard so you can wear it round your neck or attach it to your key ring, rucksack, backpack or wherever else you prefer to carry it. It's lightweight so it's not going to hamper your activities or refereeing - you can take it with you when you're camping, hiking or even just walking or cycling around locally.
Free postage & packing in UK.
P.S. It's worth installing the what3words or OS Locate app on your phone - that will give your precise location even if there's no mobile signal where you are - most rescue services recognise its code words.
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You can return the item within 30 days for a full refund. If it's faulty, I'll reimburse the return postage but if it's just because you've changed your mind then I'll ask you to pay the return postage.