It's a slow Sunday morning in snowy Salisbury...
Howmanyleft says there's 323 Aero chassis cars registered in the UK, of which 238 are taxed.

There's another 5 in the Aero model listing with engines smaller than 4 litres.
And another 50 or so in 'model unknown' Morgans with 4.0 - 4.8 litre engines - which could be Aeros, or Rover engined Plus 8s.
There's no mention of the Coupe - which is presumably included in the Super Sports or Super Sports Auto listings.
The numbers of S5 Aeros looks about right for 2015 - 2016, but 2017 (marked as a ? in HML) looks light at just 11.
The Plus 8 registrations looks right too, trickling along at 4,10,3,5,2... but then leaps to 43 in 2017.
So maybe taking Aero and Plus 8 together gives the right number for 2017 - 54, and a total of 91 Aero chassis cars since 2015.
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So what about sales & values?
There's 20 dealers in the UK - most of whom you'd expect to have an Aero or Plus 8 for sale.
Taking 3 major online outlets where you'd expect to find Aeros advertised:
Pistonheads have 14 Aeros for sale in the UK
Autotrader have 8 Aeros for sale (let's exclude Williams' imaginary used Aero GT from the numbers.
Car and Classic have 19 Aeros for sale in the UK
Some of those dealer cars for sale are unregistered.
So even if there were no duplication amongst those ads (& there is) that's 41 cars for sale, from 323 registered cars - 12.5% of the stock. Halve the number for sale to take out the dealer demonstrators, and you're at 6%.
If fewer than 10% of these toy cars are for sale, I'd reckon there's not "a sign on people cashing up in the face of the economy / brexit etc." to quote Howard's original question.
If anything, looking at the dealer prices (& knowing how the price list has changed since launch) the advertised values are often more than a dealer might have expected to advertise the car for at the point at which they ordered it.
fwiw, based on market valuations, my insurer has accepted an agreed value more than 10% over the invoice cost of the car.
The market for Aeros & Plus 8s looks very healthy.
Will