For Royal Mail, at the moment, I'm afraid this isn't something we're able to offer. However, if you do have a relationship with your local Postman then you should be able to give them any safeplace instructions.
If you have opted for DPD, once your order has shipped just head on over to your tracking and there will be options to upgrade or change your delivery date, update the shipping address, and add a safe place or delivery instructions. You can also download the DPD app, which gives you great and easy to use in-flight options.
For our lovely US customers, DHL have excellent in-flight options on their On Demand Delivery site. DHL may need to get in touch if they are having trouble locating the shipping address. At that point, you can give them more specific delivery instructions to help them out.