Is prostitution legal in UK

It entirely depends upon the nature of the agency. Some escort agencies are legitimate, providing female company but no more than this, so then this would be legal. All to often however, they are used in the UK as fronts for prostitution or human trafficking, which are definitely NOT legal.

The whole 'escort agency' phenomenon is a shady area in UK society, regarded both by the public and the police with suspicion. Legitimate agencies have to be specially licenced and are subject to regular monitoring checks by both the police and by social services.