Not sure how long they will last, but here is a free 20 pound Google AdWords coupon at services.google.com/marketing/links/UK-OA-NETCRAF found on SearchGuild.

Update: here are some more recent coupons

Google AdWords:

• You can get a free $75 AdWords coupon here (or here or here or here or here or here or here) ... many options linked because some of their coupon offers expire over time & we update this page periodically. The Google Partners Program also offers coupons to consultants managing AdWords accounts.

Bing Ads: get a free Bing coupon today.

• Are you from the United States? Use this $50 coupon.
• Are you from Canada? Use this $50 coupon.
• Are you from the UK? Use this £50 vouchers.
• Are you from Germany? Use this €50 coupon.
• Are you from France? Use this €50 coupon.
• Are you from Italy? Use this €50 coupon.
• Are you from Spain? Use this €50 coupon.
• Are you from Australia? Use this $40 coupon.
• Are you from New Zealand? Use this $40 coupon.
• Are you from India? Use this ₨1000 coupon.
• Are you from Brazil? Use this R$60 coupon.