Hello,
In case, if the registrar winds up the business, you will only loose their control panel for particular domain. If the domain is .com/.net gTLD domain then you may contact "VeriSign" in order to release and unlock the domain name. You may even contact "ICANN" if you wish to release and unlock the gTLD domain's. In regards to .UK domains, you will definitely need to contact NOMINET (
Nominet ) and get the IPS Tag changed to any new registrar. So there is no need to panic even if the registrar has folded up.