Here in the UK, our emergency number is 999. Or you can dial 112, as that's the European standard, and we'd be daft not to implement it even if we hadn't agreed to do so. Alternatively, try dialling 911, which I understand also works and which was introduced to help children who grew up with US TV.
In any case, the phone won't actually dial the number even if you do type it in -- if you dial what it thinks is an emergency number, it initiates the emergency call setup, which is what lets you dial without a SIM and on a different providers' network. So a dedicated button is all that's required.
This does mean that there's a possibility that my phone and the PSTN disagree over what constitutes an emergency network, but as the phone gets its numbers from the SIM, which comes from my provider, which is regulated by the same people as make the rules for the PSTN, it seems likely that all the options will work.
https://en.wikipedia.org/wiki/Emergency_telephone_number