Free shipping included in a promotion

[FrazerPart]

I am about to set up a promotion for a percentage discount off a range of products.
As part of this promotion, I'd also like to offer free UK delivery (we normally charge our customers for delivery)

Setting the promo up is easy, but how do I tackle the free delivery issue?

[leehack]

Unless shipping is a set fee or a set %, you can't as far as i know.