I would use whichever errs in favor of the customer. Its only one cent, after all.

If full price is £9.99 and you've got it on offer at £5.00, that's less than 50% off. If you want to say you're offering it at 50% off, you need to price it at £4.99.

In banking, I believe the convention is to round to the nearest penny, and round half-pennies to the nearest even penny.

Oh, sorry. I would say that selecting the rounding policy is up to you. I'd agree with the poster that gives the fractional penny to the customer.