As Washminster (partially) returns - the press are publishing stories claiming that the coming election will see an exodus of Labour MPs - retiring before facing defeat. These stories have been denied [see http://news.bbc.co.uk/1/hi/uk_politics/8029262.stm]- but it is worth considering the differences with House of Representatives & Senate retirements.
The incumbancy factor is not as great in UK elections as in the US. People still tend to vote by party than for a specific individual in the UK. So the retirement of an existing member doesn't make the seat as vulnerable as an open seat in the US. Followers of British politics tend to look at the national swing - which may differ slightly in individual seats.