According to This is Not Work you can pick up 10 Philips Energy Saver Lightbulbs for £1 at Robert Dyas:
Old lightbulbs are being phased out. Get over it! As an advertising slogan might put it. And where once they cost a fiver a pop, you can now pick them up for pence; 10 pence in fact at Robert Dyas. In store only.
Thanks for the tip!