Need to be a little cautious when making comparisons. The UK may well be different to many other countries (Germany for example, or USA) where only type approved (TUV etc) parts can be fitted and these must comply with original manufaturers performance specifications for emissions.
Here we don't have "in-use compliance testing" in the same way that other countries do (vehicle must meet certification standards on a full emission test), we just have an MOT check. The checks done at the MOT are pretty crude and really only tell you if the basic emission control systems are working, there is feedback, the A/F is controlling about right, and the idle and fast idle emissions are within a test limit. This bears no resemblance whatsoever to a certification test, an aftermarket cat would very probably perform way below the required level to comply with certifcation standards of a drive cycle.
The mass flows at idle and fast idle are trivial, and the amount of catalyst material required to convert the regulated pollutants in that mass is minimal compared to a cold start full drive cycle.
I don't know what the actual regs are concerning aftermarket emission equipment in the UK, I'm pretty sure it isn't regulated as such.
Engine power is unlikely to be affected, it's a simple job to reproduce the monolith cell pattern and density which is what determines backpressure. I suspect the monoliths may well come from OE equipment factories, the ceramic is the cheap part, the precious metal loading is what costs the money. Even a plain washcoat will be active to a small extent.
