Welcome to the forum
The dealers £400 kit comes with full sized alloy, rear door fitting and colour co-ordinated plastic cover, so it may seem dear but there is a lot too it. I got this as an cost extra (at £300 in 2016 spec).
As for a spare you can get them on ebay, there is a spacesaver version 15" going for £32.99, as long as the 4 hole pitch is right? send an email to seller the pitch distance and confirm. i do not see it being different from a 2017 model, but worth checking.
As for rolling radius of spare, you can use a smaller diameter wheel, as long as you stick to 40 MPH or under, and that you only use it to get you either home or the garage.
https://www.ebay.co.uk/sch/i.html?_...+tyre.TRS0&_nkw=ecos[port+spare+tyre&_sacat=0
PS.....get a good extending arm brace, about £7 to £10 out of car parts shop
and get a
2 ton capacity sissors jack, decent make is just under £20.
Couple of triangular folding metal wheel chocks out of Halfords, £5.