Marriage Visitor visa for UK if illegal immigrant

I'm trying to get the various comments into a coherent answer, even if I'm not quite on topic for the question you asked. I cannot give you legal advice, but I really think that you should talk to a lawyer or tax advisor who can, mostly because of the second item in the list.

1. As phoog pointed out, as a national of an Annex II country, you can visit the Schengen area visa-free for 90 out of 180 days if you meet some conditions. Days spend in Spain on your student visa do not count against those 90 days, but days in France or other Schengen countries and days in Spain after April 27th do. So you may not have overstayed yet.
2. I don't know how much (if any) paid work your student visa allowed in Spain. As a visa-free visitor under the 90/180 rules, you cannot work in the Schengen area, you can only talk about work to be done elsewhere. (That's a gross oversimplification, talk to a lawyer.) That means you may be an illegal immigrant after all, not because of the days but because of what you did.
3. As the stable but unmarried partner of an EU citizen who works in an EU country other than his homeland, you may be entitled to a residence card if the relationship appears stable enough to the authorities. As a married spouse, you are entitled to a residence card unless the authorities think it is a sham marriage. But either way it is a bad idea to just move to France. You need to apply for the residence permit.
4. The paperwork for a marriage (in any EU country) might take longer than the days you have left if you are now a tourist (i.e. not working) under the 90/180 rule.

It appears to me that you should travel home and then apply for the permit to join your boyfriend in France, either as a domestic partner or for marriage in France. That would be under EU rules, while going to the UK would put it under UK rules. phoog seems to differ.

@phoog, you are free to edit this instead of commenting.

The answer by o.m. is essentially correct, but instead of accepting the invitation to edit it, I am posting this answer, because we disagree on one or two key points and I don't see how I can modify that answer in a way that expresses both opinions without becoming confusing.

First, I agree with his point 2: even if you haven't overstayed your authorized period of stay in the Schengen area, it is probably not legal for you to work in Spain.

Second, his point three is partly correct and partly incorrect. After you marry, you certainly can just move to France and apply for a residence card after you get there. If you want to move to France before marrying, however, it is not at all clear how you could achieve that.

As the stable but unmarried partner of an EU citizen who works in an EU country other than his homeland, you may be entitled to a residence permit.

Actually, if you can demonstrate that you are the stable but unmarried partner, you are entitled to that permit under the EU directive. Only it's not called a "residence permit" but a "residence card," because the document does not grant the right of residence; rather, it reflects the right of residence that you have automatically as the family member of an EU citizen. This distinction is important.

The difference between an unmarried partner and a spouse or registered civil partner is that the derivative right of free movement is automatic in the latter case, while in the former case it depends on the relationship having been "duly attested." In other words, it is required that you submit evidence to the state demonstrating the nature of your relationship.

You can gain the derivative right of free movement, therefore, either by marrying/registering or by applying for an "extended family member" card. The application serves as the attestation of your relationship. If you do the former, it is clearly not required under EU law to leave the Schengen area in order to apply for the card, because, as noted above, the right of free movement applies to you automatically in that case. The penalty for not getting the card within three months of moving to France is only that the cost of the card rises from €25 to €340. There's no risk of deportation or any other severe consequence.

On the other hand, applying for an "extended family member" card is problematic because France has not transposed this element of the directive into its domestic law. It is not clear to me how one would pursue this option, let alone whether it would be necessary to leave the Schengen area in order to do so. I therefore agree with o.m. that you should talk to a lawyer if you want to pursue that.

Finally, as I see it, you have three practical options under the freedom-of-movement directive:

1. Marry as soon as possible, move to France, and do whatever celebrating you can in the UK.

2. Register your partnership civilly, move to France, and then get married in the UK.

3. Find out how (or even whether) you can apply for a "durable relationship" residence card in France, and, if you can, use it to get married in the UK.

A fourth option is to give up hope on the free movement regime, and just go back to Venezuela and apply for a marriage visitor visa. After you're married, you can move to France and apply for the residence card.