BYO for < £300 - piece of p1ss
Off the Google shelf
Mobo £45 (Asus P5N - also a v nice Gigabyte available)
CPU £40 (E2160 £35 - you can probably get some P4s cheap too if you don't mind the extra cooling)
Twin tuner £50 - (i.e. Hauppage DVB-T 500)
500 GB HDD £45 (i.e. Hitachi E7K - but honestly - get the SP1 750)
DDR2 2*1GB £25 (Corsair PC2-5300) (twice as much as you would really need)
Case & PSU £60 (Antect minuet 350 slimline case and 350w PSU)
Upgrade box to Lian-li c36 for an extra £90 for the best looking box you'll ever shove under your tv.
Personally I would also splash out for XP so that I could use GBPVR which has a better interface than any pre-built box I have used and is very stable (reboots for system upgrades only), HDMI interface with tv, full remote control, access to local network shares, daily tv guide updates that don't interfere with operability and DVD playback. Season recording (3 different options) configurable start and end buffer zones (default and per recording) and transcoding options for various standards including DivX, MPEG4, ts.
GBPVR also has the added advantage that as its win only the user forums aren't full of willy-waving pingus.
Can be quite a lot of work initiallly but I wouldn't go back to a box again given the lack of flexibility that even the best of them suffer from.
Next project - a Mythbuntu box
Alright lads - I'm going in - full speed ahead and don't spare the penguins