Personally I would never buy a "Display Model". That said having paid for "Know How" they should either repair or replace under the Sale of Goods Act. In your case the laptop "Is not Fit For Purpose" under the Act, unless they said it was sold "As Seen", though the fact that you purchased "Know How" you should be covered. I suggest you write a formal letter of complaint to Customer Services stating you require the laptop in accordance with The Sale of Goods Act to be repaired or replaced with the same or similar spec. model at no cost to you.
Xiaomi Mi Mix 2 review