If you don't know how to build a pc, you have to spend at least £500 to have a reasonably good one, a good choice is this:
http://www.amazon.co.uk/gp/product/B00C72IR5O/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B00C72IR5O&linkCode=as2&tag=yold04-21
In case you know how to assemble a pc, or you have some friend who can do the work for you, here is a list of the best computer components you can buy for £400 and let you have a better pc than the previous one:
cpu amd fx 6300 (£73.36):
http://www.amazon.co.uk/gp/product/B009O7YORK/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B009O7YORK&linkCode=as2&tag=yold04-21
motherboard Asus M5A78L-M (£48.77)
http://www.amazon.co.uk/gp/product/B0054U7HIO/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B0054U7HIO&linkCode=as2&tag=yold04-21
Crucial 8Gb 1600Mhz Ddr3 (£64.21)
http://www.amazon.co.uk/gp/product/B00A14ZT6U/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B00A14ZT6U&linkCode=as2&tag=yold04-21
graphic card Radeon R9 270X (£120)
http://www.amazon.co.uk/gp/product/B00FL8H49Q/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B00FL8H49Q&linkCode=as2&tag=yold04-21
hard disk 1tb Western Digital (£39.99)
http://www.amazon.co.uk/gp/product/B0088PUEPK/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B0088PUEPK&linkCode=as2&tag=yold04-21
power supply LC-Power 550W (£35)
http://www.amazon.co.uk/gp/product/B000ADPS86/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B000ADPS86&linkCode=as2&tag=yold04-21
Samsung 24x Retail SATA DVD Writer (£12.21)
http://www.amazon.co.uk/gp/product/B00CC6YN6M/ref=as_li_tl?ie=UTF8&camp=1634&creative=19450&creativeASIN=B00CC6YN6M&linkCode=as2&tag=yold04-21
total amount: £393,54