If you are creative/willing to DIY you can use a service like www.vistaprint.co.uk which has some templates for flyers/postcards and you can generally move things around on the page. They are cheap, but watch the postage, it adds ££ on unless you are willing to wait a month as it posted from Europe somewhere.
Or I got my Christmas cards (again using their 'desktop publishing software') from www.goodprint.co.uk/ they are UK based (I think!) so delivery is quick.
Or if you want to pay for artwork (which generally will look better I find!) then I have used www.promoworx.co.uk/ who are a local branch of printing.com and have an in-house designer.