Nominet, the registry for .co.uk domains, will be conducting maintenance over the weekend of July 28th and 29th.
From the official notification:
The first outage will be from 09:00 on Saturday 28 July and will last for one hour. During this time, all messages to the Automaton will be queued and they will then be processed at the end of this period. The DAC and Whois services will not be available. The online services, normally available through our web site, will not be available either and a notification page will be displayed in place of these systems.
The second outage will be from 09:00 on Sunday 29 July and will again last for one hour. The same services will be affected during this second outage.

Although the outages are scheduled to last a total of two hours they may last longer.