Does voluntary departure from the UK count as deportation?

It is well established that a Voluntary Return is not the same as a Deportation and even not the same as a Removal. The answer provided by Gayot Fow on Difference between deportation and removal provides a good distinction.

Further evidence of this distinction is they have different penalties.

Immigration offender: RFL05, paragraph 320(7B) and A320

RFL5.2 How long are applicants automatically refused for?

12 months if they left the UK voluntarily, not at the expense (directly or indirectly) of the secretary of state;

2 years if they left the UK voluntarily, at the expense (directly or indirectly) of the secretary of state, more than 2 years ago; and the date the person left the UK was no more than 6 months after the date on which the person was given notice of the removal decision, or no more than 6 months after the date on which the person no longer had a pending appeal; whichever is the later

5 years if they left UK voluntarily, at public expense;

5 years if they were removed from the UK as a condition of a caution issued in accordance with s.134 legal aid, sentencing and punishment of offenders act 2012

10 years if they were removed or deported from the UK;

10 years if they practised deception (which includes using false documentation) in support of a previous visa application.

In your case however I would perhaps err on the side of disclosure and say YES and add an explanation. My reason for this is that the question seems a catch all covering all kinds of deportation-like events where for the sake of brevity all the different three scenarios were not enumerated. Of course the golden rule is not to volunteer information to immigration when you're not specifically asked.

However simultaneously I view UK consular officers very intolerant of any form of deception, misrepresentation or appearance of deception and they are not people to try to use technicalities on. Some more informed people may have a different opinion and I will gladly withdraw this answer if that is the case.

When I was recently asked by immigration officer at Heathrow have you ever had any problems with immigration anywhere, I chose to answer yes although the only problem I had ever had with immigration was a visa refusal. Someone could reasonably argue that being denied a visa previously does not count as problem with immigration however I chose not to risk saying no and being refused entry and banned for misrepresentation.