The driver for this is GMs desire to get rid of its unprofitable European division. I say unprofitable, but you can never really be sure. I have had mostly Citroens and SAABS with equal enjoyment and you will recall that GM dumped SAAB a few years back. True, SAAB never made a profit, but soon after acquiring them GM "repatriated" the intellectual property (brand, patents etc) to the USA and charged SAAB a licence fee, so they moved profit from a high tax area to a lower one (USA). Much like Disney Corp and Euro Disney if the reports are to be believed. I doubt they would have stayed in so long if the European business was making a real end-to-end loss.
Still, GM Europe is up for sale and PSA probably want to jump closer to VW Group sales and turnover. I see some big problems. The van manufacturing in Luton does Renault as well, so may be divested early on. Looking at passenger cars and comparing with VW group - although VW has lots of brands and variations its fundamentally very few floorpans and engines - you can look across the range and see the equivalent VW, SEAT, Skoda and often Audi. A merged PSA/GM Europe will have too many engines and basic designs, some huge rationalization will be needed to avoid being like BL in the early days (23 different engines when Ford had 5).
That bodes ill for the UK plants, sorry to say and may put some dealerships at risk as well. If they get it wrong it puts the whole PSA group at risk. Less choice for consumers as well.