That's the thing, it is based on "reasonable belief". You'd have to stand up and convince the court that you was beyond all doubt that the person you apprehended was doing wrong. If your belief is based on flawed presumptions/lacking evidence (you can't convince the court) then you are at risk of prosecution for assault. Essentially, we can't all go around tackling people down because we think they're a "bad 'un".
I think generally the perception of our rights to citizens arrest is wildly over estimated. I think many people confuse their ability to make a citizens arrest with their right to self defence. The right to self defence applies in extreme cases though and the circumstances are usually much more clear cut as to when using reasonable force is legal in preventing imminent injury or loss of life to yourself or another person.
You seem to have a rather warped view of this.
If Person A detains Person B with reasonable force where Person A has genuine honest held belief Person B is making off with stolen goods and is acting in good faith then person B has no right to "self defence" in the way you describe it.
If Person A takes hold of Person B in a reasonable way, explains themselves and Person B attacks them then Person B is up for assault, not Person A.
I am bored of this sense of entitlement and "rights" everyone seems to think they can exercise in the extreme in EVERY circumstance. Yes, you have rights. Yes you can exercise them.
However, punching someone for detaining you with honest held belief is not exercising your rights, it is not self defence and it is not reasonable. Such action would only lend corroboration to the fact you are making off with stolen goods, for example, and likely attract a greater use of force that is now justified and reasonable in the circumstances.
I am not debating the fact 'Person A' would need to justify their course of action. Absolutely they would need to, they would also need the evidence to back it up. Be that by corroborating witnesses, CCTV or otherwise.
A reasonable (innocent) person, upon being detained, would explain themselves, present a receipt or return to the shop to clear up the misunderstanding. They might not be happy about it but they certainly would not exercise "self defence" and go crazy on shop security, nor would they have any right to.
Does being detained expose someone to fear of imminent attack/violence? Not for most reasonable people.