Palestine is a pseudo-state, Syria and its de-facto protectorate Lebanon are in disarray.
Iran does not have the breadth of interests and worldwide presence Britain has, nor a problematic (recent) colonial past. Despite this, their actions make it very difficult for them to build solid, trusted alliances with reliable partners.
Britain has a worldwide web of influence because it has proved through recent history to be a trusted and reliable partner for many allied countries. If they start pissing all over the Vienna Convention and be branded as unreliable by their peers, less people will be keen to hear their reasons on all sort of matters, and the already-waning British influence will see a dramatic decline.
It's already hard enough to lecture China as it is; if Hague "renditions" Assange, the Chinese will rightly feel entitled to simply laugh to his face whenever he brings up "human rights".
