With laptops a chunk of the purchase is for the screen, but you already have monitor. Although you are looking for Windows 7, there may still be a lot of software suitable for Windows XP.
Any more details on the Dell ? Also, have you looked at just getting the 'base' unit from a company like Ebuyer.com or www.scan.co.uk
Why the need for wi-fi ? A PCI wi-fi card should be possible for under 25 pounds but there may be other options.
Have just checked on the scan.co.uk site and found this Asus EEE box but unfortunately you probably need VGA (3 rows of 5 pins) connection for your monitor.
Revo R3610 has VGA as well, running Windows 7, 4GB RAM, wi-fi built in, and 280 quid with free delivery.
It has plenty of RAM (I have a 2 GB RAM Windows 7 laptop here which is fast enough) and dual-core so it is up to date.
Perhaps someone else might spot some better option, but hopefully this ticks all the boxes.