Was Belgian neutrality guaranteed by Britain or its empire?

Did the dissolution of the British Empire end Britain's undertaking concerning Belgian neutrality?

No. It was a British commitment rather than an undertaking of the British Empire, so imperial dissolution did not itself end it.

Britain held the undertaking concerning Belgian neutrality. The dissolution of the British Empire did not itself end a commitment made by the British state. Identifying Britain as the guarantor answers the question about whether imperial dissolution removed the obligation.