The use of a megaphone as part of a peaceful protest is lawful. This was confirmed by Justice Holland in the High Court in 2007 when he stated that:
“Protest is lawful; the use of a megaphone as an adjunct of lawful protest is itself lawful. The starting point is unfettered freedom to engage in so much amplified protest as is neither intimidating or harassing.” HLS Group Plc v SHAC 2007 WL 919475  EWHC 522 (QB) QBD.
Use of a megaphone does not constitute a “section 5” offence (s5 of the public order act 1986), it is not Breach of the Peace (see other articles) under the CRIMINAL LAW section.
However, Section 62 of the Control of Pollution Act 1974 states that there should be no loud speakers in a public space between 9pm and 8am.