You could always call the agency, but other than discover how willing they are to speak to you, there can be nothing gained from asking an agency if they are ‘legitimate’. Whatever their line of business may be, no company would want to discredit themselves to a potential client, so if they were not a ‘legitimate’ company, then the likelihood would be that they would readily tell you everything you wanted to hear.
The length of time an agency has been in operation is usually a good indication of their success and check with Companies House, as good agencies will be registered as a Limited company.
Although there are no regulatory bodies to control agencies in the UK, all companies must operate within the guidelines of the DTI (Department of Trade & Industry). I refer to them as DTI still, as they have had many name changes (BERR, BIS). Any company not seen to be working within their guidelines should be reported immediately. This does not necessarily mean that they are acting illegally, but the DTI will ensure that they are not doing so, whilst checking that they are a bona fide company.