From 'Citizens Advice'
You can keep the goods
The Consumer Protection (Distance Selling) Regulations say you have a right to keep goods delivered to you that you didn’t ask for. But if goods are sent to you by mistake, you need to contact whoever sent them to let them know and ask them to collect the goods. You might get goods sent by mistake if they are meant for someone else or you’ve been sent duplicate or extra items on top of what you ordered.
If you receive goods you have not ordered and which haven’t been sent by mistake, you can treat the goods as an unconditional gift and you can do what you want with them.