In summary, no, not as a routine. During an initial Class 1 examination blood haemoglobin levels will be checked (fingerprick test) and cholesterol level. A urine sample will also be required but this is to screen for blood, sugar and protein in the urine (which could be indicative of problems with the kidneys and/or bladder and diabetes for example) rather than any STD.
See
Initial Class 1 (Professional Pilot) Medical Examination | Medical | Personal Licences and Training and
http://www.caa.co.uk/docs/1859/20111...tsAndATCOs.pdf for more detailed information on what to expect.