My recommendation MaBook Air 13 with 4gb RAM and 256gb Drive (top model) £1,129 with VAT
Go to an Apple store and have a play/look at the various options side by side, staff are very helpful and knowledgeable.
Rationale. The MacBook Air is super portable, light and thin with very long "all day" battery life. It is very fast due to SSD - so instead of a traditional spinning hard drive it has a special type of ram which holds information when machine is off. It is also very cool, sorry couldn't help myself !
The MacBook Pro is more directly comparable to a pc in terms of weight and would be the choice if you wanted more storage, ram and hard drive on-board the machine, you can also pay a premium for the retina screen which is outstanding but possibly overkill, personally I'd have the lightness and speed of the Air over the Retina screen but others don't agree. As above you can add SSD to Pro's. The 13in Pro (non retina) is £1,249 and has much more memory and storage than the Air above. The £100 you save with the Air you can spend on an external storage drive / backup
As you are a business you should have a backup, so an external hard drive or possibly cloud storage. Backing up is trivial and automatic with Mac's via TimeMachine which comes with the computer. If you need/want/afford you can buy an Apple combined wifi router and 1TB backup / storage device, but you should have something perhaps upto 500gb if you want backup and additional storage (eg for lots of movies, video files). The backup/storage device doesn't have to be an Apple product, I use a 640gb "free agent" portable drive.
Maintaining a Mac means just clicking ok to software updates when they appear from time to time. Trivial. Even Operating system updates and major upgrades (which are rare and cheap and optional) are done this way. The downside with Air is that upgrading ram and storage is troublesome, so buy the most you can now. Normal MacBook you can change hard drive (inc for solid state) or memory easily. I am still using my 2009 MacMini and I only recently upgraded it from 2gb to 8gb ram as it ran ok with what was the base spec back then.
If you don't have an iPhone consider one next time as Apple products work very well together. Assuming you buy before Mavericks the new operating system comes out very soon make that upgrade, should be pretty cheap or even free. Apple TV is a small box which turns your flatscreen tv into another monitor is wireless (needs mavericks) and allows you to stream video/music/mid week mini movies from your MacBook or iPhone to the tv (you may be able to do this with a single £25 cable also with new Mac's, my old machine needed cable plus separate sound). My setup is Mac Mini desktop (usually connected to tv but also used with standard widescreen pc monitor) plus iPad for portable device.
In my view Macs are much more reliable than windows machines, better hardware and windows just grinds to a halt after couple of years due to accumulated garbage. After 2 or 3 years you can sell you Mac for 50% of what you paid for it whilst your windows laptop goes in the bin. Alternatively you just keep using it as it's still working well ! If you buy an Air and you don't like it you can sell it to me (for a discount of course )
Any more questions drop me an email.