This document is all about road signs and markings and the keep left/right arrow is in Section 4, Compulsory and Prohibited Movements. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/782724/traffic-signs-manual-chapter-03.pdf "4.7.1. The “keep left” (or “keep right”) sign to diagram 610 (S3‑2‑3, see Figure 4‑25) may be erected without an order. It is used at traffic islands, refuges..." That sign is the same (white arrow on a blue back ground) as arrows which indicate one way streets and round a bouts and no one would consider going the way opposite to what the arrows indicated there. The keep left/right arrow is the same and to not comply with it is an offence here, Sec 36 RTA 1988; http://www.legislation.gov.uk/ukpga/1988/52/section/36 "36 Drivers to comply with traffic signs. (1)Where a traffic sign, being a sign— (a)of the prescribed size, colour and type, or (b)of another character authorised by the [F2relevant authority] under the provisions in that behalf of the M1Road Traffic Regulation Act 1984, has been lawfully placed on or near a road, a person driving or propelling a vehicle who fails to comply with the indication given by the sign is guilty of an offence."