Best laptop for multi tasking

I have been trying to find a laptop for academic purposes.

Needs to be able to handle the following when being used most intensively:
20-30 google chrome tabs statistical software package
1-5 word documents
5-7 open folders
a few pdfs
referencing software
windows live mail client
searches across 30+ pdfs for keywords.

The machine needs to be very efficient and not suffer too much lag, I get seriously irritated by slow laptops.

Hard drive space, built in webcam and battery life are not important. Ideally it would have good cooling and won't burn my legs as it would spend a lot of time on my lap.

The biggest challenge is:

I have a very tight budget of £600!!! Can anyone suggest a good laptop?