Switch all the other registries over to the Nominet system!
Maybe it's just me, but IMHO the Nominet method works SO much better than the others, most notably with domain transfers. Firstly, unlike the system with the GTLD domains like .com etc, where to transfer the domain the receiving registrar has to request the domain, and then the holding one has to accept the transfer, use a Tag system, so the holding registrar passes the tag to the new registrar. That disposes of all the faff they've had to put in place with domain locks etc to stop people poaching domains.
Secondly, allow the registrant to go direct to the registry when the registrar fails to act on a request to transfer the domain.
We've had loads of clients who have wanted to transfer domains to us, but have registered their domains with people unwilling or unable to action the request. With .uk's it's simple, just get them to go direct Nominet, who will then (once due dilligence checks have been made to confirm ownership) transfer the domain for them, circumventing the unresponsive registrar. Yes they charge a fair amount for the priviledge, but in a situation like that it's a small price to pay.
With GTLD domains there's little or no way to transfer the domain if the registrar fails to respond to the request. If you try contacting the registry you're simply told to contact the registrar (well duh! tried that already!) as the registry doesn't speak to registrants direct.
The upshot is that you can safely register a .uk domain name with any registrar, safe in the knowledge that if they turn out to be dodgy there is an escape route available. With any other domain name you better have done your research before hand, because if not you're screwed if they go under or just decide to prevent you from leaving.