Do take someone with you. I would be extremely surprised if they'd allow you to bring a friend, your partner or a solicitor. They have said a union rep or colleague, and that is what you are entited to, nothing more.
Do take someone though. Apart from anything else, they can take notes, enabling you to concentrate on what is being said.
You mention wanting compensation. The only way they are likely to give you anything financially is if they think you are bringing a tribunal claim. Otherwise why would they? They might pay you a small sum of money to get you to agree not to bring a claim, but otherwise there's no reason for them to do so.
If you were to bring a claim for constructive dismissal, it would be extremely hard to prove and would be stressful. Any compensation awarded would solely be based on your financial loss, nothing to do with how badly they've behaved. You will also be expected to continue to look for work, although if you are signed off as medically unfit for work obviously that wouldn't apply. If you do find alternative work, your financial loss won't be very much so your compensation would only be very small, making it fairly unlikely to be worth it.
DOn't want to be a doom and gloom merchant, more giving you a bit of realistic thinking so that your expectations aren't too high.
Definitely take either a union rep or colleague with you. For this I would advise a union rep if at all possible tbh.