Concur with fobs.
I think the conventional wisdom is that if the powerpoint serving your computer is switched and both it and the lead serving your computer are earthed, then you are best off leaving the power cord in place and plugged in, but with the powerpoint switched off. Keeps everything grounded, keeps the volts away and you only have to touch the chassis prior to handling sensitive bits to discharge any static you might be carrying.
I regularly do it this way here in Oz - much the same household power standards as UK, I believe. So far no grief.
Unearthed 2-wire household supplies with no switches at the powerpoint, as found in USA and Canada, are different - I would follow Richard's advice re unplugging the power cord in that situation.
Cheers
AA