I feel that there will be a very strong used market for these cars, especially lower milage models that haven't been used as Ubers, etc. Sure, everyone wants the latest and greatest model with all the bells and whistles, but the reality is people have a budget and a £15k second hand EV which is family friendly might be the ideal car for many. The same way that a £15k Toyota 'hybrid' doesn't hang around long on 2nd hand market, the MG5 is likely to be the new model for that demographic. More cities will get low emission zones, more people will want to move to EV when replacing their ICE cars, I can't see the market shrinking.
You can almost guarantee that a huge influx of newer, better models will impact current residuals - but it's not as if we're tripping over these cars just now. There are only a few thousand in the whole of the UK (I have no idea of the real number). Time will tell - I'm not too fussed either way, I'll keep this for 5 years or so and move to whatever brand/model is best suited to me at that stage.