My understanding is that production and imports will be strictly limited.
So if the car is a hit, it may mean high residuals and difficulty getting one second hand. If it is a big hit, people may even pay over list to get one in the short term.
I think this could be a very different MG experience from that which we are used to, so I wouldn't assume large volumes, rapidly declining prices and easy to pick up second hand cars after 6 months.
After PCPs/leases end, 2 or 3 years in, there should be more of a secondhand market. But it depends how long people want to wait.