The fitting looks like a type of push-fit tap connector. The nut on the tail piece of the inlet valve unscrews in the normal fashion, the plastic part of the fitting will probably have some sort of release mechanism, which after being activated will allow the fitting to be pulled from the pipe.
If you want to remove the fitting completely and can't get it off, just cut through the copper pipe under the fitting with a junior hacksaw, then use an alternative fitting to make good again afterward, such as a 15mm-1/2" flexi tap connector or a 15mm-1/2" Speedfit tap connector.
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