You used to be able to make emergency calls in the UK without a valid SIM card, but it's been required since 2009 [1][2] (apparently to cut down on untraceable hoax calls).
There is a technical difference though, between 'Roaming' and using another network in the same country. I have roaming enabled on my Vodafone account, but I can't connect to O2's UK network when Vodafone is out of range, because the other UK networks are on a list of not-permitted networks. The technical solution is called LSS - "Limited Service State" (sometimes referred to as "Emergency Roaming", and that's what allows us to 'camp' on another network to make an emergency call, but it's a different technical implementation to roaming.
Here's a thought though…if someone visits the UK from abroad, and hasn't activated roaming, is their SIM card considered 'valid' enough for the sake of LSS/Emergency Roaming?
[1] http://community.giffgaff.com/t5/Using-giffgaff/Emergency-Ro... (GiffGaff is a UK mobile operator, but the requirement stands for all UK operators).
[2] http://media.ofcom.org.uk/news/2009/next-steps-to-ensure-ava... (Ofcom is the UK regulator for communications industries).