we don't know whether he was "fighting" the black belts on the ground or standing. I'm going to assume on the ground as it's much easier to catch an elbow or get something to the mask of the face on the ground than standing. The OP would have to get back to us to tell us what was going on.
The OP didn't mention ukemi, but then he didn't mention a ton of stuff. As for "fighting" black belts - it is common for new students to think this is really fighting, and that they are being matched up with heavier black belts for the purpose of being cannon fodder. That's rarely the case - black belts and higher level students are usually assigned because they are safer - they can control their techniques and prevent a new student from hurting them. Sometimes it takes a while for a new student to settle in and realize that he's not getting his ass kicked, and that if he settles down and works the technique that he's been taught, the upper belts will help him. Spazz out, and the answer is different. Treat it like a fight and not ronduri, and the answer is different. And yes, the more a new student spazzes out or has difficulty with doing the techs without over doing them, or risking his own safety, the more apt I am to pair him with heavier higher grades. I'd much rather pair a new student who's not picking up on things well with a larger student than a similarly sized student or a smaller student, assuming all are the same skill level. The last thing I want is an injured student, and a larger student usually prevents harm when it comes to such new students (mentality has some to do with it as well, but you know what I mean).
Listen, we don't know a lot of stuff. We don't know how many similarly sized students of brown or black belt level were available to roll with the guy. We don't know what context the 5 pound bit was in - if you're in the middle of the class, watching people randori, and explaining things to a new student in his first class, explaining that the classes cost 5 pounds - it would be odd to have a student whip out a 10 pound note and ask for change then and there.
We don't know what's happening. Whatever is happening, the OP's not having a good experience, and he doesn't feel like he's being heard by the instructors or other students. Going to a different dojo would be the best bet. Going to a few different dojo, if that's an option, and shopping around for the type of experience and dojo he wants makes the most sense.
But, OP needs to understand - you're going to be paired with higher ranks - they keep you safe as you get your feet under you. They are not there to kill you. You should not be paired with a similarly ranked judoka until the school's comfortable with your ability to fall, take a fall, and basic understanding of technique. You'll randori with them on the ground usually before you get to your feet.
OP also needs to know that we don't fight in class. Randori is not about fighting, it's not about winning.