Which one's a better deal?
When compared the similarly priced Macbook Pro to the iMac, the iMac is much (much) more powerful. (much

)
For £1249, you get a Macbook Pro retina with a 2.4Ghz i5 Dual Core (w/ Hyperthreading), 8Gb of RAM, 256GB SSD, Intel Iris GPU, and a 13" retina screen.
For £50 pound more, you can get an iMac with a 2.9Ghz (turboboosts upto 3.6Ghz) i5 Quad Core, 8Gb of RAM, 1Tb HDD, nVIDIA GeForce GT 750m with 1Gb of GDDR5 VRAM, and a 1920x1080p 21.5" screen.
So the iMac has a larger screen, a much faster CPU (almost double the geekbench score), a much better graphics card (Iris is nothing when compared to the 750m), you get VRAM.
The Macbook Pro has a higher resolution (although I would argue that the Iris GPU still is not good enough to run such a high res display while doing intensive tasks), and an SSD.
Seeing as a second monitor could set you back a good few 100 pound (well, it depends on the size, res, and if it is Thunderbolt, pre-calibrated [properly] or IPS).