Under the Town and Country Planning Act 1990 it is illegal to display an advertisement (a word that captures these posters) without permission punishable by a fine of £1,000 plus £100 a day while to contravention continues.
There is a presumption that the owner of the structure or the person who benefits from the poster (more relevant when adverting goods, services, or organisations) is the one who did the posting.
If it wasn’t the owner then, technically, they have trespassed against the owner.
The owner of the structure or the person who posted it can lawfully remove it.
Under the London Local Authorities Act 1995 the local borough may give notice to either of the above to remove it and, if they do not, the borough may remove it and recover their costs.
Anyone else removing it is theoretically committing criminal damage against the owner of the poster and trespass against the owner of the structure. Neither is likely to be prosecuted.