A levy applied only to white British people would immediately face legal challenge under the Equality Act 2010, which prohibits direct and indirect discrimination on grounds of race. Section 13 of the Act defines direct discrimination as treating someone less favourably because of a protected characteristic. Race is a protected characteristic. A tax that white people pay and others do not is direct racial discrimination by any reasonable reading of the Act.
The government could not implement such a levy through primary legislation without repealing or disapplying the Equality Act for this specific purpose. That would itself be a profound political and legal act with consequences far beyond the reparations question. It is not a realistic prospect.
