Just in case any of you carry a cellphone around for emergency use only, be warned:
There are some stories around which suggest that you can use a cellphone here to call the 999 emergency service even if it is not fitted with a SIM card.
This is simply NOT true.
The confusion appears to arise because the GSM specification for the phones requires them to attempt a 999 call using any available network even if no SIM card is installed. Unfortunately, none of the British network operators at present will accept such a call.
Thus if you have no SIM card fitted, you will not be able to call 999. The phone will try to connect to any available network, but the call will simply fail with a "No network coverage" message.
Probably doesn't apply to most of you I know, but forewarned is forearmed as they say.