What limits the Prime Minister's (UK) power?

Firstly, parliament is full of elected officials who have their own personal mandates and therefore will act in the way they feel represents their constituents. This means their own party may not be loyal to them (e.g. over Theresa May's Brexit deal) and even more importantly the opposition can be very powerful and at times where there is a slim majority or none at all (e.g. following the 2017 general election) the Prime Minister will struggle to pursue her agenda. The PM is also blocked by precedents e.g. she can't go to war without a parliamentary vote. The PM will also be vulnerable to public opinion given the longest they can serve before a general election is five years. On top of this there are external limits for example treaties and organisations like the EU or NATO.