The UK is a terrible place for challenging anything potentially remotely conceviable as a national security concern in journalism. I think Assange said it has the highest concurrent number of active media gag orders of any nation.

Regarding the mainstream news cycle and from a PR perspective, you want bad news to come out late Friday to minimize exposure. The typical audience is ready for the weekend and paying less attention. Also staff journalists tend to be off which leaves a weekend crew to man the ship---they're less experienced and have fewer resources available. Since you're likely not going to be pressed for a comment until Monday, the extra days might mean the public outrage subsides by then, or another news story becomes more important.

A lot of bad news comes out late Friday, government reports, financials, etc. It's obvious that the Washington Post was sitting on this particular story and decided to post it only after today's Guardian reports. WaPo has done it quite a lot since the leaks came out, either they aren't particularly interested in breaking these stories, or they're abiding by an earlier deal with Snowden since they wouldn't agree to his release timeline.