A move to the EU is going to be much easier than emigration to somewhere such as America or Australia. As a British citizen you can move freely around the European Economic Area, settling wherever you see fit without need to apply for visas or undergo rigorous interviews or points based tests.
However, whether you are moving to Belgium or moving to Holland it is still not as simple as getting a plane ticket and a passport, and there are still a number of things that you need to ensure you do before you head off towards that sunset.
Before heading off, you need to ensure you talk to HM Revenue & Customs regarding your liability for tax. The laws will differ depending on where you are moving to so don't just assume that moving to Belgium will carry the same rules as if you were heading to Spain. In fact, you should spend time researching all the laws for the given country that you plan to move to so that you are fully aware of both your rights and the things that could potentially land you in trouble.
Fully understand the situation regarding healthcare. Again the benefits you will be allowed will differ from country to country and it is essential that you are adequately covered from the very start of your life abroad.
Finally, you need to let people know you are going. Even for those who simply want to disappear and get away from their current life, there are certain people who will need to be informed of the dates you leave from schools and banks to any outstanding creditors that may well need paying after you move. You also need to remember to forward your post so that any important letters reach you no matter whether you are moving to Holland or the furthest reaches of the desert.