Need some help.

I'm in the US so I don't have these prices in GBP, but here are my suggestions...

A $999 13 in MBP should work fine for you. When you run out of screen space, you can add a $29 DisplayPort/Thunderbolt to VGA adapter and pick up an external 1080p monitor for another $150 or so.

You might find yourself wanting Adobe Creative Suite and that should run fine on any MBP. If you really want more screen real estate, you can go with the 15 in MBP with the higher resolution graphics.

Whatever model you get, get AppleCare. Things spill in dorms. It happens. My son had his 13 in MBP motherboard replaced under AppleCare after a spill saving us the $900 repair cost. Normally they don't cover accidental damage but they made an exception for my son.

 

Another vote for the 13" MBP. The 17" is big and not really that portable (13" is very portable). Buy a monitor for extended screen area. Load it with 8GB RAM.

The 13" model will have major improvements in performance in the next release, probably a month away. Less so for the higher models.

Buy Parallels or Fusion and a copy of Windows 7. This allows running windows software alongside OS X. You will need to run Internet Explorer if nothing else.

 

r0k was very lucky. AppleCare does not cover accidents. I has a white MacBook that got wet when it was a bit more than a year old. Not only did Applecare not cover it, I was quoted $1200 for the repair on a computer that was only $1000 when new! I found a place in NYC that offered guaranteed repairs for water damage. With all the shipping included I got it fixed for about $300, but lost all future AppleCare coverage. The computer is now over 4 years old and still running, needing no further repair work.

 

Another vote for the 13" MBP. I have the 13" with the i7 and its an amazing machine and plenty powerful for what you need it to do. Since you already have an external monitor there isn't much of a point in getting a 17 as its a behemoth and not very portable. The 13 will serve you very well, and even better if you add 8GB of RAM and an SSD. You can use the money you'll save getting a 13 to add the RAM and SSD and still have some pocket change left over! Go for the 13 it will do everything you want and more. As far as software goes, almost everything is compatible with Mac, and if a program isn't there is usually a MAc equivalent.

 

Yes, the 17" is big compared to the smaller MBPs, but its pretty thin and light compared to other 17" laptops, and its got more pixels than your 1080p screen.

Consider the 17" if you want a portable, self-contained system for web development that you can easily move from a to B without sacrificing screen space.

Consider the 13" if you want something you can carry around easily for basic WP, web, email, presentations and demonstrations and "emergency" development, but which you normally hook up to a large monitor for serious development work.

If you're undecided, 15" is a very nice form factor...

As for web development, the Mac is great because you have a Unix system which comes with Apache, PHP, perl, PostgresQL and that can natively run most of the usual free and open-source suspects plus native versions of adobe CS and Microsoft Office if you can't afford them. Personally, I install MacPorts and use that to install custom builds of Apache and PHP to make it easy to add options and modules - but if you're targetting commercial webspace the built-in versions will probably have the generic features you need. The web debugging tools in Safari are great, too.

(I tend to use Eclipse and the only caveat is that some of the newer, bleeding edge add-ons haven't been tested properly on Mac, but the basics are fine).