The 2012 13-inch is interesting, as it was sold new (still being manufactured) well into 2016.
It's the last Mac sold with an internal optical drive (that slot can be used to add a second hard drive/SSD)
And, it's that last Mac laptop that can easily upgrade RAM. (up to 16GB)
It's the last Mac sold new with a Firewire port.
2012 is the first year that all Macs came with USB 3.0.
And, because that 13-inch was sold new only 4 years ago, it is still fully supported by Apple for repairs, replacement parts, etc. The 15-inch, OTOH, is classed obsolete (no support or repair parts from Apple now)
The only true value is what you can get someone to pay for it.
I think £400 is maybe a little high, but maybe not outrageous.
If you are the seller, ask for more.
If you are the buyer, less than that is probably good!
(web sites I checked that
buy those old Macs will buy that model for $150 to $175 (£133.60), regardless of the configuration, and they don't seem to accept the idea that an 8 year old laptop could possibly be in "excellent" condition.)
I also saw a 2012 13-inch MacBook Pro for sale as a used Mac for about $1620 (more than £1235 (!) )
Looking for a new or used Mac? OWC can help you look at price points for used Macs so you aren't paying the full price for a brand new Mac.
eshop.macsales.com
I don't know who would pay THAT...