From this site - http://www.environmentlaw.org.uk/rte.asp?id=49:
In England and Wales, graffiti is considered an act of criminal damage under the Criminal Damage Act 1971 and offenders can be punished with an unlimited fine.
Is it illegal to deface graffiti? For example, spray painting over existing graffiti on a public wall.
By defacing I mean black-boxing, drawing a line through, painting over with the colour of the wall. So not tagging, but just spoiling or covering.
It depends...
The statutory defence relevant to the OP may be found at s.5(2)(a) of the Criminal Damage Act 1971 https://www.legislation.gov.uk/ukpga/1971/48/section/5?timeline=false
if at the time of the act or acts alleged to constitute the offence he believed that the person or persons whom he believed to be entitled to consent to the destruction of or damage to the property in question had so consented, or would have so consented to it if he or they had known of the destruction or damage and its circumstances...
So the burden of proof is on the over-painter to establish that they believed they had consent to commit further criminal damage (by over-painting black boxes etc).
If it is you can put whatever you like on it (subject to planning permission) - you can paint it a single colour, create an artwork or sign your name over and over. It’s only illegal graffiti if it’s done without the owner’s permission.
