The You Choose Party (YCP) plans to replace first past the post with the additional-member system for UK general elections. The additional-member system is already being used for the London Assembly, the Scottish Parliament, and the Welsh Parliament.
The YCP would apply the additional-member system to UK general elections by taking the 650 existing parliamentary seats and distributing them, proportionally according to electorate sizes, amongst the UK’s twelve regions: South East, London, North West, East, West Midlands, South West, Yorkshire and the Humber, East Midlands, North East, Scotland, Wales, and Northern Ireland. Then, in each region, half the seats would be constituency seats and the other half would be additional-member seats. (For regions that have an odd number of seats, there would be one more additional-member seat than there would be constituency seats.)
At a general election, electors would be asked to cast two votes. The first vote would be for the individual that the elector would like to have represent his/her constituency. This would be the same system that the UK electorate is used to for general elections. Typically, however, due to the additional-member seats, the constituencies would cover bigger areas. (Existing boundaries would be used to create these larger constituencies.) The second vote would be for the party that the elector would like to have govern him/her. And it would be the second votes that would be the ones used to determine how the 'additional members' would be allocated amongst the parties.
The D'Hondt method would be used to allocate the additional members amongst the parties, and this method would be applied once for each of the twelve regions. This method is the same one that is used for elections for the London Assembly, the Scottish Parliament and the Welsh Parliament. It's really just a way to ensure that the additional members are allocated to the parties as consistently as possible with what the voters wants. Typically, there would not be a perfect match between what the voters want and what they get, due to what's called the electoral/effective/hidden/informal threshold, but the two would be more-or-less aligned.
Right, so that determines, for each region, how many additional-member seats each party would get. But how would each party decide how to fill those seats? This is done using a list of names, one for each region, that the party has to publish in advance of each general election. If a party is awarded three seats in a region, it fills those seats using the first three names on its list for that region. If a party is awarded ten seats in a region, it fills those seats using the first ten names on its list for that region. And so on.
This is how the YCP would apply the additional-member system to UK general elections. In essence, the system would maintain the constituency link at the same time as ensuring that total final seat allocations would be consistent with what the UK voters would want.
The following flowchart shows the paths that the YCP could take.