What did the University you will be visiting say when you asked them?

@mlc believe it or not they are a bit unsure and most of them are travelling right now. I am in hurry as I have to apply soon to get the visa before Sep 1st.

Have you followed the UK government website Check if you need a UK visa? After answering a couple of questions (as if you) there was a lot of information, and whether you need a visa at all seems to hinge on whether you are being paid.

@WeatherVane I am bringing my own awards and scholarships with me and will not receive funding from any UK source. The problem is that I cannot demonstrate any course enrolment with my UK university (as there is none). I am confused with the scope of the 'academic visit' and do not know if it also includes visiting Ph.D. researchers visiting for less than 6 months.

Yes but, why do you need a visa at all if you are a Canadian citizen on a non-commercial visit? If I say the reason is "Study" it says "You will not need a visa to come to the UK". If I say the reason is "Work, academic visit or business" it says "You do not need a visa for some business and academic activities, but you must get a visa if you plan to work in the UK". Obviously you'll need to bring some evidence to present at the point of entry.

@WeatherVane I should have clarified; I am not a Canadian citizen. I am in Canada with a study permit and also need visa to enter the UK.

Then please state your nationality in the question. That website says "What’s your nationality as shown on your passport or travel document?" The country you are arriving from isn't relevant (except a few cases).

@WeatherVane I am Iranian.

Oh, in both cases it says "You’ll need a visa to come to the UK". Sorry for wasting your time.

@WeatherVane Not at all. Thanks for taking time. It's just that I don't know the subcategory of the visitor visa.

Permitted activities for academics are PA 11.2. :Academics may: (a) take part in formal exchange arrangements with UK counterparts (including doctors); and (b) carry out research for their own purposes if they are on sabbatical leave from their home institution. Does that cover your situation/what you intend to do?

@Traveller Not really, as I am not participating in a formal exchange program, nor am I on sabbatical leave (that's for faculty members). But I think this for the 12-month academic visitor visa. The activities for PA 11.1 may be suitable though: (a) gather information and facts for a specific project which directly relates to their employment overseas, or conduct independent research. I don't know if that goes under the 'academic visit' option or if I identify as a researcher.

@KFK Since you are not eligible to apply under short-term study, applying as an academic is the only option. See also Science, research and academia in assets.publishing.service.gov.uk/government/uploads/system/…

What did your host supervisor advise when you asked them about the appropriate visa category?

@Traveller He doesn't really know the exact specifications of the visa categories. But the responsible person in the Department initially said that as I am going as a visiting Ph.d. student, the 'short-term study' is the route to select. However, since I am unable to demonstrate a course enrolment and have been engaged with the university as a visiting researcher, she said that the visiting academic route would be the most appropriate. She was really unsure though.

@KFK Am I correct in assuming you will not be working (paid/unpaid))? travel.stackexchange.com/questions/87730/…

@Traveller correct. This is mentioned in my visiting association offer and also the host supervisor visa letter: ‘ except for the permitted activities listed above, the person will not undertake any form of employment in the UK’, and ‘ You will not be an employee or a worker of the College during this association and will not be entitled to receive any remuneration from the College’.

@Traveller "Since you are not eligible to apply under short-term study": how did you reach that conclusion?

@phoog The OP’s conclusion, as stated in the question

@Traveller the question contains no such conclusion.

Sorry I rolled the question back: anonymizing Iranian as International isn't helpful.

The OP is apparently studying to acquire a PhD (e.g., "...I am an Iranian doctoral student" and "...to complement my PhD studies..."). If this is so, why does the question title refer to a "PhD researcher" ? The phrase "PhD researcher" suggests a person who has already received the degree, not one who has not yet obtained it.

@WeatherVane The specific country of nationality isn't necessary to the question. All we need to know is that the poster is a visa national.

@DavidRecallsMonica you are right it’s misleading. I have corrected the title. I wanted to emphasize on the research nature of the visit.

Additionally, the nationality is still relevant, as well as the domain of study. You may need an ATAS certificate before commencing your research activities in the UK, and the process is unfortunately more complicated for people coming from countries the West don't like (for just or unjust reasons).