S&P doesn't use a fixed schedule. I emailed Standard and Poor's Rating Desk to ask, however, and they confirmed that:
There is no set schedule for the updating of our Sovereign Credit Ratings. Furthermore, these ratings are reviewed at least once every 1-2 years, and the reports are updated whenever a review is completed.
They also sent me a PDF with information on how they rate sovereign debt. I believe the actual PDF is only available to users that register (their website says registration is free), but you can find a cached version on Google.
In the section titled "Process For Rating Government Issuers", it says:
For government issuers, Standard & Poor's rating process is typically initiated when the issuer or its representative requests a rating for a particular debt issue. ...
Most sovereign issuers have rating agreements with us, but some sovereign ratings are classified as "unsolicited" because we do not have a rating agreement with the issuer. We continue to rate these sovereigns when we believe there is significant market interest in them and there is also sufficient public information of reliable quality to support our analysis and ongoing surveillance.
The portion I emphasize probably explains why ratings were published more frequent during the height of various debt crises; not only is the market more interested in credit ratings during such periods, but new information relevant to market conditions was continually coming to light in some cases.
On their rating list, S&P states that their ratings for the UK are unsolicited. I asked for clarification on this and they responded that although there aren't any set dates for such reviews, they usually take place "once or twice a year." Ratings are published once the review is completed, which according to the document linked above, "typically takes four to six weeks to complete, but can run longer or shorter."
Short answer: You can probably expect new ratings to be published about twice a year for the UK because it's a sizable economy and major issuer of sovereign debt, but apart from that, S&P doesn't follow a set schedule.