Im not sure what precludes you from doing web design, web development and programming on a PC but anyway...
Out of the two I would probably go with the MacBook Pro but as others have suggested it would be worth getting an external monitor with decent resolution, but it sounds like you've decided on the iMac. Good thing about the iMac is even though it's a desktop it's still very portable for when you have to shift it around as much as a student does.
I only really use my PC as a Gaming machine now, I don't like working on it for some strange reason, that's what I use my mac for, it's just feels, I don't know? More friendly.
I do have a nice 21.5" Monitor here with a resolution of 1920x1080, but if I were to carry my PC around all the time, i'd have to bring everything with, the speakers, kb, mouse, tower, etc.
With an iMac it's all built in so it'd just be the iMac + kb/mouse which seems more logical to me.
In the back of my mind, i've always wanted a MBP but getting an iMac, with all that power? I think that's the better option.
The iMac.
You're not going to be taking the MBP to lectures realistically. It gives pretty much no benefit over a pen and paper to justify the worry of carrying around a 1k piece of kit. The only benefit of the portability would be occasions where you might want to goto the library (but there's computers already there no doubt), or if you wanted to take it back to your folks at the weekend.
It's really awkward doing any real work on a MBP. The iMac 21.5 allows two full size A4 sheets side by side (iirc), which is my benchmark for a machine capable of doing serious work on.
To be honest, I think you're right, more often than not i'd be in a rush to get to uni as i over sleep sometimes ;_; lol, and well grabbing the MBP would be the last thing on my mind, not to mention the fact that I wouldn't really want to carry around a £1000 laptop or rather be afraid, incase i knock it or whatever.
I like to work with windows side by side one another, now while this would be accomplishable with a screen, that would already be a factor I wouldn't have to worry about with getting the iMac.
I think the iMac is going to be the viable option here and also, I get a student discount and ALSO, i'm going to be trading my Mac Mini in for apple store credit or just selling it on e-bay, so i should be able to get the iMac fairly cheap.
Comes with thunderbolt now too and a quad core? It'd be like i'm on my PC gaming machine but on my Mac, I'd never have to switch, I think my mind is made up.
iMac 21.5" here I come :>