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I tried that and got a machine for £1535 with a GPU that's twice the size. Could beef it up to 32gb of RAM and still not get to £1999. Whilst MBPs have their merits - a high end future proof laptop by apple just costs so much. I think the size sacrifice is worth the power if you need it for serious work. Photoshop isn't very demanding at all, doenst require much GPU power and will idle along on an i5 quite happily. It's when you get to 3D stuff that things get graphically demanding. That being said I'm anti laptop anyway. I can't justify the cost to get the performance I need. That being said if i was going to invest in one it wouldnt be a Dell. Again, you pay for the name somewhat.
I'm no apple fanboy, but you're not really giving the full picture.
For example, go to the Dell site and configure a Precision 7510 to a similar spec to the top level MPB (£1999) and you get a price of £1850 - that's with some weird 30% special offer.
The benefit of the MBP is that they are still portable, they run reliably well and fast ( I'm currently working on a 4.7GB Illustration in PS on a 2013 MBP - no lag at all, even when scrolling through RAW files)
You can build a Windows based laptop to perform well, and possibly better performing with CAD/3D but it'll be a lot less portable and it will still be expensive.
Nice windows machines are Lenovo ThinkPads - very well made and thought out machines with good customisations available. XEON chips, massive GPUs etc. But it'll cost about £3-4k for the top end machine.