Basil,
The FCO is limited to giving advice as contained in the FCO travel Advice. Now that may seem tautological, but the FCO Travel Advice is a cross-government piece of work reviewed as often as necessary by seniors from FCO, MOS, Home Office, plus the Security Services. In my previous role I would actively contribute to it, sometimes daily, but would also get enquiries from MOD and PJHQ, saying 'yes, but what's it really like?'. This one agreed security advice avoids the chaotic system the US has, whereby DOD, COCOMS and State Deparment can give wildly differing advice.
Some of you will know a colleague of mine (RAF) who ignored FCO travel advice very recently, and narrowly missed being involved in a terrorist incident with his family; indeed they passed the gunman as he walked up the road to attack a nightclub!