A de-cat will fail your MOT due to emissions, and if police were to ever stop you to do a random spot check you may be in trouble there too. Best bet is to get a sports cat but they are literally 4 times the price (for the Milltek anyway). I am saving for a sports cat as it eliminates the need to swap it over for an MOT, and doesn't mean you have a light on your dash (until you get it bypassed) and there won't be much lost in regards to power, possibly 4 or 5 bhp? It's your call