That's not right at all. Loads of stuff is inadmissible as evidence but not illegal. For example, in many countries phone intercept is inadmissible. For example, in The Wire, they might hear a drug deal being arranged on the phones, then arrange to be on patrol with a drug dog and find the stash. The phone recording would be inadmissible (even though it would help the prosecution), the evidence used to prosecute would have to be paralleled.
If the evidence they're hearing on the phone is not admissible, there's a good chance it's because they're listening through illegitimate means as well, hmm?
In the UK, they do not have the Fruit of the Poisonous Tree doctrine. Parallel Reconstruction is not needed to bypass admissibility constraints because those constraints don't exist in the first place.
Parallel Reconstruction is used to avoid scrutiny of the original source of evidence. Even when used to legitimate ends (and I have doubts how often it's used as such), there are fundamental problems with dodging accountability.
Parallel construction is still used in the UK. That's my point - it is used to conceal sources in a wide variety of circumstances, and is orthogonal to any issues regarding illegality of evidence.
