Why does season 2 of Gurren Lagann start so differently compared to season 1? Episode 1 of  the second part of season 1 of Gurren Lagann features more political themes than those seen in season one. However from episode two onwards it returns to being more about action.

Is this to do with how they were aired? Was this change enforced, so as to move away from the Mecha stereotype, by a 'higher up' in Gainax...and simply ignored from episode 18 onwards? In western TV, ratings that are seen by executives can influence the show...has something similar happened here?

Why is this?
Your question appears to be premised on a bit of a misunderstanding of how anime in general (and Gurren Lagann in particular) is produced. 

The two so-called "seasons" of Gurren Lagann (ep 1-15 vs. Lord Genome, and then ep 17-27 vs. the Anti-Spiral) were not actually discrete seasons. They were aired together as one continuous block of 27 episodes (with episode 16 being a recap of the first arc), from April to September of 2007.1 As such, any changes in theme that you may have perceived immediately following the time-skip cannot have anything to do with there being a break in the show - because there was no break! (Besides the one week for the recap episode, anyway.)

Furthermore, it isn't really possible that any changes in theme could have resulted from public reaction to the first arc of the show (ratings/etc). According to folk wisdom,2 a typical 1-2 cour anime spends over a year in production before it airs, meaning that the vast majority of work (storyboarding, character design, art, etc.) is done by the time the show starts airing. Given the timeframes in question, it isn't feasible for a studio to make significant changes to an anime during a season. 

Between seasons, of course, anything is possible - the classic example of this is Code Geass, which saw some major changes in its second season, in part due to a change in its airing time slot. But during a single few-cour season, there are serious limits on how much the studio can change on the fly.3

In sum, whatever changes you may have noticed after the time-skip were almost certainly present because that's how the producers planned for it to be; not because of external factors like viewer opinion or high-level execs or whatnot. 



1 Note that a single "cour" of anime will almost never be 15 episodes in length, which is another tip-off that the two "seasons" weren't separate.

2 If you have something better than folk wisdom, please answer this question!

3 Note that none of what I have said applies to super-long-running anime like the big shounens (e.g. Naruto, One Piece, etc.). I don't know anything about what production cycles look like for them, but it's pretty obvious that they have to be running all the phases of production in parallel, meaning that they should be able to react to viewer opinion (though I have no idea whether or not they do, in practice).