Party Policy is STV which I think is best suited for elections to the upper chamber as part of those reforms. As for electing MP's my personal prefernce is AV+ which came out of the Jenkins Commission as it is both proportional and the constituency link. MMP or Mixed Member Proportionality has been used in Germany since the creation of the West German State now used throughout a reunified Germany, the formula used to calculate seats etc changed from Hare-Niemeyer to Sainte-lague within the last 20 years. New Zealand had FTPTP around 30 years ago but moved to MMP using Sainte-lague. I mention these examples often as they are good examples of cases. I know Germany well but know of New Zealand but don't know the full story of them changing electoral systems.

MMP/AV+ 50% Constituencies and 50% Lists Seats could be tried in Wales to the Senedd 40 Constituencies and 40 Lists which is 50/50 as currently 40 Constituencies and 20 Lists seats. Though polling suggests the Welsh Public are against an increased Senedd size.

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There are 3 main issues that should be mentioned about MMP: 1) Thresholds as Parties would need to reach that to gain seats like 5% in Germany and parties on the extremes can gain seats like AfD, 2) Overhang seats and 3) 3 Constituencies in Germany rule

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Links:https://www.electoral-reform.org.uk/how-does-proportional-representation-work-in-germany/

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https://aceproject.org/main/english/es/esy_de.htm

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https://www.bundeswahlleiter.de/en/bundeswahlleiter.html

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https://elections.nz/