This is just not happening. $300 (converted) is just not enough to do anything with when building a machine. The absolute cheapest you could get the major parts (CPU, mobo, RAM) is going to be about $175-200. The cheapest case you could find is about $30-40, and the cheapest decent PSU would be about $40. An optical drive is about $20 as well. Right there, you're already over your budget, and you still need the HDD ($50-60). It's just not happening.
I highly recommend you look at some prebuilts from the discount vendors like Dell and HP. They're not high quality, and they're not ideal, but you can't beat their prices.
However I do not know if any of these products are easily available or how much it would cost to ship/deliver them in the UK. Also I made a quick glance to see if everything fits so it's not yet done thoroughly.
The case has a psu so no need for another one (though I have no clue if it's any good).
Theres this http://www.ebuyer.com/product/253549
or even better £1.30 over budget all parts from ebuyer.com
Asrock N68C-S UCC GeForce 7025 Socket AM2+ VGA Out 6 Channel Audio MATX Motherboard 221178 £30.36
Seagate ST3250318AS 250GB Hard Drive SATAII 7200rpm 159691 £27.15
Casecom KB-7760 Black ATX Midi Tower Case - No PSU, No Case Fans Included 108280 101 in stock £10.98 £10.98
Sumvision 450W 20+4pin SATA Power Supply PSU - Retail Boxed 120674 £14.99
LiteOn IHAS124-19 24x DVD±RW DL & RAM SATA Optical Drive £11.99
EXTRA VALUE 4GB (2x2GB) DDR3 1333MHz Memory Kit 1.5V CL9 £29.55
AMD Athlon II X3 445 3.1 GHz - Socket AM3 1.5 MB cache Retail Box Processor 235421 £56.28
Cart total inc vat: £181.30
The power supply is a bad make but its all you can get on the budget.