Yes, you can buy it after, but I'm pretty sure you can't buy it for the £45 it costs when you buy the actual machine. if I get the MBP I definitely wouldn't get a screen until september, but the initial cost will still be £1050.
The question is again, do you want portability or not? Why did you sell your iMac? There's no reason to buy Mini if you just sold your desktop so I would still go for MBP. Portability is always needed