You probably need to spend a bit on the lappy but not that much on the desktop. Photoshop is RAM-hungry, but it's a 2D app and so doesn't make heavy demands of a graphics card. What monitor are you thinking of? I have a Dell 24" monitor which gives me plenty of screen space for working and these days doesn't dost the earth click here
A lot of photographers use Mac laptops. You could, for example, go for this click here and perhaps spec it up a bit to a bigger hard drive; and plug it into your monitor when you're at home to give you more screen space.
Add to your shopping list an external hard drive something like this click here as your backup; and your copy of Photoshop (for Mac if you buy that laptop) click here and I'd chuck a copy of Lightroom into your shopping basket while you're at it - lovely piece of software click here. That would give you a total spend of around £3k for a really nice pile of kit and software.