Sorry, but (assuming that you're a British citizen, travelling from the UK) you must have a passport to travel to any other country (except the Republic of Ireland). The only exception is if you live or work in the North West of England (or work airside at London City Airport - see below.
The European Union has always permitted free movement between its member states upon the presentation of either a passport or a national identity card but, since the UK has yet to introduce ID cards for the majority of the population, that means that you require a passport to enter Belgium unless you meet the rquirements (outlined above) for a £30 ID card:
http://idsmart.direct.gov.uk/index.html
It was always planned that passport controls would be abolished across the whole of the European Union. This has now come about across 25 of the 27 member states, as well as in a few other countries, under the Schengen Agreement. If you were starting your journey to Belgium from any other Schengen country (such as Spain or Latvia) you wouldn't require a passport or ID card to enter Belgium (although the carrier might still demand ID, such as a driving licence, on security grounds). That would apply irrespective of whether you were Spanish, Latvian, British, Australian, American or anything else.
The UK is the ONLY member of the European Union which refuses to become full signatory to the Schengen Agreement. (Ireland would like to sign but can't unless it closes its border between the Republic and Northern Ireland). Our politicians are too scared of the potential outcry in newspapers like the Mail and Express if they decided to join Schengen. So (unlike the citizens of other EU countries) you're forced to buy a passport if you want to travel abroad.
Chris