Everyone will recommend the G&G Combat Machine M4s / G&G MP5A4. You could buy a transparent G&G CM16 (VCRA-compliant, so no defence needed) from Pro Airsoft Supplies (excellent retailer) for £100, or if you have a defence then Zero One Airsoft sell them too.
ICS Sportline guns are similar to them and around the same price I believe.
You will however need to buy a battery and charger too (there should already be a lot of threads on this so you don't need to start a new topic, but Component Shop are a great site to take a look at)
If you can though since you are new to airsoft I would say try to go to a local shop if you can and get a feel for different guns. There might be a particular firearm you like the look of but you might not actually like the feel of it if you haven't handled one before.
Hope this helps, and welcome to the forum.