Yes BSB's creche is the only place in the region of Brussels and just outside which is registered and has mother tongue English speaking carers, from 18 months, there is also International Montessoris.
Is there a reason why your child cannot go to French creche or Dutch creche? It really won't harm their English if you speak English at home. Even if for a short time and they forget the other language if you move away, there is much evidence of exposure to another language being beneficial.
If your child is 18 months old, they could start school this time next year and it's almost free in French or Dutch. There are loads of after school activities in English in Brussels and just outside, a huge percentage of the population speaks English as a first or second language. Dutch is the official equal language of Brussels region but in fact only spoken by around 8% of the Belgian population. The percentage of fluent English speakers in Brussels region is probably larger than 8%. The areas of the region where the upwardly mobile English speakers live are east and south of the region, but more and more moving to the northern communes due to the European school 4 being deliberately located in Laeken, however the amount of activities in English in the north is still limited.
BSB's creche (for under 3s) moves building in November 2014, to a portakabin but a luxury one, with much new wooden furnishings, should look like a Montessori type place I imagine, it will be a huge improvement, no more stairs, easy to go outside, near to the lower primary facilities (3-8 years). Yes it is very expensive, but there is nothing else like it. It is however only open for 38 weeks a year, so you'd still need something for the other 14 weeks.