Holiday visit to UK with a C-Visit visa which I got for a business trip that never happened; meantime I left the company that backed my application

So, this September I applied for a Standard Visitor Visa in order to spend 3-4 weeks in the London office of the company where I worked, and in the application I said I would stay 3 weeks for a business seminar. I had an invite & employment letter to back the application. I was supposed to go in November, but in the meantime a lot of things happened at work and I left the company, and I didn't use the visa.

If I wanted to go to London next month just for a few days (so not 3 weeks, as stated in the application) simply for tourism purposes, would that be a problem?

I read on some of the previous questions here that if your main reason for applying was business-related, your first visit has to be a business one, but all these questions were prior the changes to the visa categories. And how will they know if you're entering the UK for business or travel, except your answer to that question?

So, I want to know when I enter the UK whether someone will ask me if this is a tourist visit, even though the initial reason stated on my application is business, and if that will be an issue?

What you described comes under the generic term, "change of circumstances". This occurs when the premise of person's application is no longer valid. This is a different case from a person who obtains a visa and then uses it multiple times for business and pleasure.

When you applied for the visa you signed an agreement to keep them informed about a change of circumstances; you are also obligated to tell the IO when you arrive if there has been a change of circumstances. If you use deception by silence (per the Immigration and Asylum Act 1999) to not inform them, you risk becoming an illegal entrant.

When they learn of a change in circumstances, the critical question the ECO (or IO) will ask himself is: "would the change have altered the decision to grant the visa?" If the answer is 'no', it's not significant enough, then your visa is still valid. If the answer is 'yes', then the visa is no longer valid.

The controlling technical reference for this is Paragraph 321 (ii) of the Immigration Rules, which says...

A person seeking leave to enter the United Kingdom who holds an entry clearance which was duly issued to him and is still current may be refused leave to enter only where the Immigration Officer is satisfied that:

a change of circumstances since it was issued has removed the basis of the holder’s claim to admission, except where the change of circumstances amounts solely to the person becoming over age for entry in one of the categories contained in paragraphs 296-316 of these Rules since the issue of the entry clearance; or

Based upon what you wrote, it's reasonable to conclude that your visa is no longer valid, but if you have doubts there's three paths you can follow...

1. arrange a consultation with a solicitor; or
2. use full transparency in your landing interview and hope for discretion in your favour; or
3. apply for a new entry clearance.

Of these choices, the safest and least expensive is option (3).