There is only one mainstream political party in the UK which could broadly be described as "centrist", the Liberal Democratic Party, formed by the unification of the Liberal Party and the SDP in the 1980s. Having said that I think most LDP members probably think of themselves as slightly left of centre.
The LibDems are also the most strongly pro-EU party and really anyone who does not support brexit ought to be supporting the LibDems.
However there are strong vested intersts, particularly in the media (and that includes the BBC), which make it their business to denigrate the LibDems at every opportunity. This is the party which made the very difficult decision to join the Tories in coalition in 2010, only to be targetted ruthlessly by them and their friends in the media in 2015.
If you agree with the Liberal Democrat's policies, or even most of them, and want to see an end to confrontational two party politics, please join us, or at least support us.
www.libdems.org.uk/why_join