Unless Apple gets their s*** together and gives us back USB3.0 connections, I will never buy a new MacBook again.
I went from a MacBook Air 13" (8gb RAM, 128gb SSD, barely 2gHz processor) which was adequate to learn the basics of ProTools on, then swapped with my wife to a 2015 MacBook Pro 13"(8gb RAM, 500gb SSD, 2.2gHz processor I think).
When I started an MA in studio recording, I did a lot of research and decided to jump in the time machine. I went used and bought a mid-2012 13" MacBook Pro with a 2.5ghz Intel i5, preloaded with 16gb RAM, and a 256gb SSD. I then bought a hard-drive caddy which you mount a drive to and put in place of the DVD drive. I put a 1TB SSD in there and run the two drives together as one Journaled drive for 1.25TB of storage, more than enough to store all the loops and stuff associated with Logic.
The mid-2012 machines were the last Macs that were totally user-serviceable. You can add or replace RAM, the hard drive, battery, fan, you name it... in order to make the machines smaller, they went with pre-soldered RAM and whatnot after this model.
The display isn't nearly as nice as the Retina stuff, and it's a touch heavy, but it cost me just shy of $500 for the used machine (I shopped around and waited to find one with some of the upgrades I wanted already, like the maxed out RAM and a SSD), plus $270ish for the 1TB drive and a bit more for the drive caddy w/tools for replacement. I'm into the machine about $800US (~GBP 600).
The savings of going for one of these older machines can be put into upgrades, an external drive for backups (buy this [b]first[/b]), and a monitor for those really intense mixing and editing sessions.