When I came over to Europe from the US, I looked for a job first and then had my employer to do the visa stuff for me. But that was a while ago, and I suspect the rules have changed since then.

What I usually suggest is that you make a couple of vacation trips over first in order to scout out the job market for your profession. You can follow the market to an extent by referring to one of the Sunday London papers (often available a few days late in a bookstore or library) and scanning the employment ads. But to contact potential employers, you need to be available on site to take an interview should one develop - which usually means you have to be ready to book a trip at short notice. And the best way to interest an employer in your services is to have something "unique" to offer that make it worthwhile for an employer to want to go through the hassle of hiring a foreigner - experience, languages, etc.