How strange. It does depend where your trial s bring performed and whether it is being performed to international GCP standards. ICH E6 (R1) is always a good place to start (always a good read).
1.56 Subinvestigator
Any individual member of the clinical trial team designated and supervised by the investigator at a trial site to perform critical trial-related procedures and/or to make important trial-related decisions (e.g., associates, residents, research fellows). See also Investigator.
1.34 Investigator
A person responsible for the conduct of the clinical trial at a trial site. If a trial is conducted by a team of individuals at a trial site, the investigator is the responsible leader of the team and may be called the principal investigator. See also Subinvestigator.

The UK SI STATUTORY INSTRUMENT 2004 No. 1031 defines investigator as:-
“investigator” means, in relation to a clinical trial, the authorised health professional responsible for the conduct of that trial at a trial site, and if the trial is conducted by a team of authorised health professionals at a trial site, the investigator is the leader responsible for that team;
“authorised health professional” means—
(a) a doctor,
(b) a dentist,
(c) a nurse, or
(d) a pharmacist;

I don't see anything in UK legislation to say that a sub-investigator must be a consultant!

The EU is making things less clear with the new EU Clinical Trial Regulation 536/2014 where they define investigator and PI but not sub-I. Consequently a “‘Principal investigator’ means an investigator who is the responsible leader of a team of investigators who conduct a clinical trial at a clinical trial site”; and an “‘Investigator’ means an individual responsible for the conduct of a clinical trial at a clinical trial site.” The reason why this is so puzzling is that they have decided against a sub-I and they use the word "investigator" twice. Hence everyone is an investigator when there is a team lead by a PI!. Still that is still to come and there may well be explanatory guidance..