Hi Kintoa,
Congrats on your new addition firstly.
I live with a 'diversity' expert and he deals with questions like yours day in, day out so I asked him what he thinks. As there is no religious requirement for dreads (we are presuming you are not rastafarian) or even a strong cultural 'norm', his feeling is that a legal challenge would be unsuccessful. Also if it is stated in your uniform wearers rules then you could be said to have agreed to it when you joined. If not though, it could be one managers interpretation in which case your best approach would be to trial it internally as a grievance against that individual. You could try to demonstrate that there were no hygiene issues around your dreads and that you undertook to have them regularly redressed in order to keep them immaculate for work.
Or you could just enjoy looking at the pictures of when you had them and get started on a baby brother or sister!