Ok, DAN C, not reading 5 pages of comment so if this is already covered , sorry.
All cars depreciate 50%ish after three years. MG4 is meant to be better than that say 45%ish but let's just stick to 50% for now. This 50% drop in price is not linear. It is probably 25% drop in year one, 15% in year two and 10% in year 3. Most cars are like this. You buy new because of many reasons but you will be hit like this. The other 50% is distributed over the rest of the life of the car. This is why a 10 year old car is still not free but probably has lost 80%.
If there are any special reasons then the depreciation might be less. One reason may be is it is Effing expensive to start with. So a £250k rolls might drop by only 40% in 3 years but it means it has lost £100k !!!!
Anyway. If you are worried about the depreciation DO NOT BUY a new car. You can shop around for a 1 year or 2 year old car which has done the big part of depreciation already (But then you don't know how it has been treated).
My own feeling is that in 4 to 5 years time people will realise that these cars can do 100-200k miles easily and so the depreciation will be minimal after the first 50%.
As far as our MG4s go I expect an SE SR at £26k should be worth 21k (19% drop from new, DFN) after first year. Then 18k (30% DFN) after the second year and 16k (38%DFN ) after year three.
I Hope this helps your anxiety