I agree with Welshmum that this is very difficult for you when you thought you had made your decision.
This is the problem when you ask for advice and opinions

you get so many different views!
I think this goes to show that there is not one right answer and that no particular set of circumstances will lead to the same result each time. This applies to lots of things in life - but in the performance industry, where supply (of actors) outstrips demand so dramatically (excuse the pun), then the potential outcomes of a particular way of trying to get work are so numerous as to be almost totally unpredictable. It's because the actor is the product - and every actor is different, so you can't train and market each actor the same way and get the same result. Not only that, but the need for a particular type of actor will be present one week and not the next. It's not like selling beer, or perfume or cutlery sets...
It's not like trying to get a job in computer sciences or engineering either where, if you equip yourself with the most marketable qualifications, you can significantly improve your chances of getting work. Just because you graduate from a top drama school and are good, it doesn't mean you will get any kind of steady employment.
I did a post-grad (one year) course and we did get some agents and casting directors coming to our showcase - however, we shared the same showcase as the three year students, so this may have been why. Lots and lots of people did NOT get agents though - they had to work at that after the showcase. This is true of most student showcases to be honest. Even at those where there are lots of agents in attendance (e.g. RADA), students don't necessarily get represented straight away.
I agree with the CD that you are giving yourself the best chance to be spotted by doing three years at a good drama school BUT, it doesn't guarantee anything and NOT going to drama school doesn't mean you won't do well.
I also think that there is something that is so often forgotten in these debates (University v drama school) and that is that drama schools do provide training! So you graduate (hopefully) as a more accomplished and knowledgeable actor than you started - therefore you are also probably more employable. This is the main reason that CDs and agents go to productions and showcases at CDS schools - because they expect to see good/marketable actors. They may also go to other hotbeds of talent and they will also go to plays and film festivals and so on. CDs and agents know that drama schools are not the only places to find talent - but they obviously provide a very
good pool of talent because a) there has already been the very thorough filter of auditions to start with and b) the actors have spent years working at their craft full time.
There will always be cases that contradict (and back up) any advice or information you are given. There are actors who attend Universities and get spotted (and not just Oxford and Cambridge). Matt Smith went to UEA for example (though according to Wiki he landed an agent through NYT). There are plenty of actors who don't get picked up though. There are actors who do three years at a top drama school and are launched straight away by top agents and do well and there are plenty of actors who
don't do well straight from drama school.
I would recommend talking to your agent. If they are prepared to keep you on through University (or keep a place for you) then you will graduate with an advantage that many other graduates will not have. You also already have a professional CV. If your agent is a good one and they believe in you, then they won't want to lose you to another agent later - so they may well be prepared to wait for you.
I still think, from your posts, that your gut is telling you to study History at University. Gut instincts are usually good ones to follow. You could apply for one year courses after that. If the funding situations are still the same then OSD for example has the potential for funding for a one year course even if you have already had funding for a degree.
The difficulty will come if you struggle to get work when you graduate - you will not be able to be sure whether this is because you didn't go to drama school. However, if you go to drama school and don't enjoy it, you may wish you had studied History - especially if you don't get work when you graduate.
I know this is a rambling and inconclusive post - and I do apologise for that - but (apart from my tendency to ramble) this is because there is no single correct answer to your dilemma.
I didn't have this problem with DS - this was largely because he didn't feel he could do anything else. DD, however, who is good at many different things, did have trouble deciding what to do and has changed direction a few times - this is not uncommon. In fact I think the "single track" view is the uncommon one and it brings its own advantages and disadvantages.
What do your parents think? Are they encouraging you to do History?
How do you see your life in 5 years time or 10 years time? Where would you like to be? Whose career do you aspire to emulate? Do you see yourself as an actor? As a film actor? A theatre actor? Can you imagine touring and putting up the sets and lights and living out of a suitcase? Have you thought what you might be able to do to earn money in between acting jobs?
Remember that your decisions now do not have to dictate the rest of your life. It may help to take the pressure off yourself if you remind yourself of that! Sometimes you do have to plunge in and do something before you know whether or not it was the right decision for you. You are being very wise to try to weigh up all the pros and cons before making a decision, but in the end you do have to take the leap.
Good luck!