Is it worth self-funding a PhD to attend a top 10 university?

Unless you're independently wealthy I think it is always unwise to do an unfunded Ph.D. program. With the current academic market the way that it is, there's just no guarantee that you will get a good job with the Ph.D. that will allow you to quickly pay off your debt. There's also no guarantee that you'll get a Ph.D. at all. We're talking about over a hundred thousand euros! There are some advantages to a top ranked department (as ff524 says, university ranking is irrelevant), but it's not 100K euros worth of advantage (especially since often a lot of the advantage of a top dept. is that they're better funded and so students can concentrate more on research).

Congratulations on receiving these two offers from good universities, both of which offer at least some funding. I make it that the difference between your two options is about €114000 over the three-year period that UK PhDs are usually funded for. This is probably a very large difference and it might be worth trying to ask somebody within your field who knows both institutions and departments whether that would be worth it.

Certainly, self-funded students can survive PhDs in the UK, and your prospects of successfully completing are probably better than average if you are in engineering and tech. In addition, it may be a realistic prospect in the UK to receive payment for carrying out some teaching and demonstrating. I would advise asking about this possibility before you commit to option B. Look at the research council guidelines beforehand to try to gauge the best practices for salary and training.

However, there may be more important criteria than money. In one department I worked in I saw hoards of both funded and self-funding students abandoned by a particular supervisor before they quit. Don't let this be you. The (prestigious) department did nothing; it was absolutely buyer beware. So the usual advice applies: Talk to your supervisor's previous students, did they complete on time? What are they doing now? Did they see others around them succeed and being supported? Did they feel the supervisor, department and university helped them to succeed? Unfortunately, a top department is absolutely no guarantee of a non-toxic research culture in the group.

If option A is research council funded (or is funded by a high-profile UK body or another funder demanding results for their money), this would weigh heavily in my decision to take it, even if it is at a less prestigious university. There will be consequences for the group and department if a council-funded student fails to finish by the four-year deadline, and this means that both the supervisor and institution are absolutely committed to the student succeeding and solutions will have to be found if things start to go wrong.

While what you should opt for is clearly a personal decision, my stance is the following:

You Should Never Take An Unfunded PhD Position

There is already a decently high opportunity cost for getting a PhD - adding substantial amounts of debt (or erosion of savings to it) just makes this opportunity cost worse. It will put tremendous pressure on you coming out the other end into the job market, and generally speaking I've never encountered an institution that didn't use unfunded positions as a sort of "soft rejection" signaling mechanism.

The hope may be, of course, that you come, prove to be an outstanding researcher, and can find funding with a member of the faculty, but that's a hope, and one that's far from guaranteed.

Especially considering that your second, funded option sounds like a very decent choice of school, I wouldn't do it unless you're genuinely independently wealthy to the point that all of this is a purely theoretical discussion, but my guess is that's not the case.