With caveat this is hypothetical, people aren’t always good judges and so very much a starting point (and doesn’t take account of pacts etc) what seems to happen is a remaining chunk of Labour’s progressive vote peels off and the party cannibalises Green-left voters. 
Support for reduction spans political divides, but Reform UK and Conservative 2024 voters are more likely to say there should be significant reductions (NB corrected data labels, previous swapped Con/Lab -apols!) 
We are a long way from a General Election, lots could shift between now and then, but it would be a mistake not to consider Reform a serious contender for government as things stand, winning in Doncaster, Runcorn, Kent, Staffordshire shows breadth of their support.
Other policies in high cut through high negativity cluster - disability benefit cuts, farmers inheritance tax and prisoner releases. This is the policy combo that along with perception of slow pace of change on eg NHS/cost of living that have led to disillusionment with new govt 
Views of politicians went beyond healthy cynicism ranging from them not getting it to being useless to being actively corrupt. This wasn’t a partisan thing, it applied to everyone Tory, Labour, Lib Dem, Reform, being part of the political class was more telling than party. 
The full seat totals from the MRP are
The number of people who say that they expect Labour to improve the lives of people like them has almost halved since the election. In July the public split 54%-46% in saying that Labour would improve their lives, that has now flipped to 29%-71% 
While clear majorities across the countries polled believe defending Ukraine's sovereignty is at least somewhat important the UK stands out for how relatively unpolarised our voters are on that question - even Reform voters are much more likely to say it is important than not. 
Who are Progressive Activists? They make up 8-10% of the public but have an outsized impact on political and cultural debates because of their higher engagement, they're more likely to be a graduate, have a higher income and are the youngest segment (though many are over 40) 
New Reform supporters are different in other ways, they are less online & their politics leans more moderate than those who backed the party in 2024. They're also less likely to hold positive views of other figures who have support on the online right such as Musk and Robinson 
The danger for Reform has always been that they listen to their very active online base who have very different views from the pool of potential voters that would grow them into a potential party of Government. Today’s announcements risk falling into that category.
Reform have momentum, but we don't know if they have a ceiling. With the exception of one pollster which has a tendency for higher Reform shares (including vs the GE) Reform haven't opened up a lead - that may change, but an open question if they can consistently poll above 25
The belief that what happens in Ukraine matters to the UK spans voters of different parties - a majority of Reform UK voters believe a Ukrainian 'win' of some form is important. That much lower Ukraine polarisation makes the UK reasonably unique among western democracies. 
But what is most striking is the gender gap. Reform UK now lead among men, but the Tories have a 7 point lead with women. On the left - Labour does better with men, but more women go to the Greens and the Lib Dems. 
Keir Starmer’s personal net approval rating has sunk to -39, the lowest we have ever recorded for him. However, Leader of the Opposition: Kemi Badenoch’s approval has dropped to -18, also the lowest so far, with Farage at -17. 
In the past Farage has recognised this and said of UKIP’s later embrace of Robinson “It goes against all the things I did as leader to say we will talk about immigration, we will talk about the extreme forms of Islam [and] we’ll do it as a non-racist, non-sectarian party”
