It is possible that she joined the Civil Service i.e worked in a government department and progressed to a senior level but access to employee records is subject to restrictions and relevant ones are unlikely to be in the public domain for many years still.
The foregoing makes it very difficult to trace her unless she died in England because unless you are lucky and find her on some electoral register there is no clue as to where to start looking. With a death certificate you might look for a death notice in a local newspaper and/or an account of the funeral and go from there.
The phrase in service is used in English English to mean she became a servant. This can cover a multitude of things from lowly maid to a companion treated almost as one of the family and could even include a nursemaid. If your relative did well for herself I would guess she possibly inherited from an employer.In this case the same avenues as discussed above remain your best hope.
Finally I should add that if you did find an area in which she resided a letter to the editor of a local paper has often been known to pay dividends. You would of course have to reveal a name to make any sense.