There are protests being organised throughout the country in support of Black Lives Matter. You can find details about ones in your local area via social media.
Make sure to check that the protest is genuine - in the vast majority of cases it will be but it is not unknown for groups who are against causes to organise disruptive ‘fake protests’. Ways to do that are to check that reputable groups such as Black Lives Matter, local political groups or other groups you trust are involved and promoting this.
Liberty has a great guide on practical tips for attending a protest during the Coronavirus lockdown, and Green and Black Cross also have some key advice for when going on a protest.
This video on protest law in 60 seconds also gives a quick and helpful guide to your rights when protesting.