Boris Johnson is within the rules in becoming an MP. The link shows that according to the GLA, “….. there is no legal or constitutional impediment to Boris subsequently becoming Mayor Johnson MP (Update, 12:20).
By becoming an MP as well as Mayor, Borie ends up with less salary
Because there is such a large financial penalty – (according to the rules, Boris must forfeit two thirds of his Mayoral salary, in his case £98,000) as an MP and also Mayor, Boris will earn less than before. Boris will lose two thirds of his Mayoral salary, which is £143,000. He gains £67,000 as MP, so ends up earning £115,000 in total, not £143,000.
What about his seat at Cabinet meetings?
Boris has pledged that he will finish his Mayoral term, and will not accept a Ministerial job until then, May 2016.. He will attend political cabinet meetings, held once every fortnight, and will not take any salary. So Boris will be doing a lot more work for £28,000 less p.a.
Ken Livingstone kept his job as an MP for two years while Mayor. The PM and the Cabinet are all MPs as well as their ministerial jobs. They all hold important, time consuming jobs, and are very good MPs.