Over here as long as it passes the MOT every year (inspection) then you’re fine.
Most of these cars will pass that emissions check with one cat and some heat - so make sure you drive hard on the way to the test and get the cat hot 😁

I think now though if an MOT tester sees that a CAT has been deleted then it’s a fail

I am aware of people who de-cat new cars as they have the three year period (soon to be four if you follow the gov's recent crap review of the MoT process) and enjoy the additional fun. Given the rules around car exhaust volume now in place it needs something to provide aural enjoyment. How the Aero dual sidepipes ever pass is a mystery to me.

Once into MoT they are getting very fussy now. A couple of GTR owners I speak to have a section of the exhaust made up where the cat has been removed. Each year prior to the MoT they slip the cat back in and take the test. Once approved it comes back off and the blank is fitted again. The engine map is selected for no CAT and the thing sounds just naughty. One did try and get buy with a hot engine and tight engine map but it was failed.

Everyone needs a friendly MOT man

The replacement bypass pipe is a good option if you don't have a friendly tester, or 4 sports cats I guess but that seems expensive..

I guess they other way round is to just bash out the insides of one and then you don't need any new bits, and visually it looks the same.

I was never a very good mechanic a lifetime ago but here goes. Don't bash the insides out, if you must take the box off cut with a grinder from the middle out so you can see the baffles then access the box and remove the plates carefully when the box is empty weld a plate back to the box. This is best done on the top of the box so when it is re fitted it still looks stock from the underneath.Be aware your engine may run a little leaner and therefore hotter whip your plugs out after a tank full and you should have your answer. Personally if you have sports cats and a sports exhaust I would leave well alone but we all have choices.It will sound good though...

Also if its a stainless exhaust weld back a stainless plate back on unless you did a good job with the grinder so the original might just fit.

I’m not going to cut or bash an existing CAT just in case it needs to go back on.

An MOT tester can’t see anything untoward as it’s all hidden on an aero

Sorry Viper should have read the post more closely I thought Nick was talking about the box not the cat. My cat was clearly visible from under the car well half of it was...,

So have you done it yet..