You're looking to advertise your web design services to a uk audience? I think this website is probably a good target market if you ask me
Other than that, have you considered offline advertising in a web design magazine such as .Net?
Prices vary from site to site, I charge $50 for a month long campaign where you will recieve anything between 30,000-70,000 impressions depending on how many advertisers we have rotating. The iNet network (people who own webhostingtalk, hotscripts, yaxay etc) charge $200 for 10,000 impressions....so the cost can vary alot!
If you're looking to work on seo then go for text links on pr5-6 pages, make sure they're site-wide and targeting your keywords. I recommend
www.good-tutorials.com for a good deal on links! If you're looking at converting sales and branding your business then banners is ideal.