It's a criminal offence to leave your home "without reasonable excuse".
The law lists certain specific activities (such as seeking medical attention, going to work or undertaking exercise) which are automatically classed as falling within the definition of a 'reasonable excuse'; it's unlikely though that your proposed trip would fall within them.
However the list is 'non-exhaustive', meaning that circumstances not included within the list might still count as a 'reasonable excuse'. It would be for the police, in the first instance, and then (if necessary) for the courts to decide what might thus qualify as a 'reasonable excuse'.
So if your roof is leaking badly and you've just managed to buy a job lot of roof tiles on Gumtree in order to repair it, the authorities might well view your trip as a 'reasonable excuse'. However if you've simply bought a new dress on Gumtree, your trip would almost certainly not be seen as a 'reasonable excuse' for leaving your home.