To be honest its not expensive dig it out, hire a mixer, use rebar 4x2 shuttering, and a good concrete mix, 2 chaps with beer and sarnies could smash that out in a weekend easy.
I would not bother getting concrete delivered for that size
I did a 10m x 5m @8 inch deep for the carport had the concrete (C20 volumetric mix on site) plus fibres delivered so that was about £950 for the concrete, do the math there was a lot of it! No MOT base for me just DMP sheet on the compacted dirt.
car port build thread
blimey that was a few yards of concrete, lol , indeed would be a nightmare if u was just using a mixer
funny enough watched this video the other day where a guy built a metal shed for his cars , was a good watch