I can't really imagine a situation where it will be cheap enough to post something that you couldn't basically shift yourself. Lockers have size and weight restrictions, and other courier services aren't really designed to provide storage until you pick it up, though if you sent it the day you left yourself, you'd almost certainly beat it down there.

Usually, the cheapest way to move things when it's too much to do it yourself is to hire or borrow a vehicle and maybe a driver, and lug it yourself. Obviously the closer you can get to only paying for fuel and food - a willing mate with a car etc - the better.

Shipping stuff just isn't really a cost-effective solution, especially domestically, because moving it yourself is usually so cheap anyway. You can take more on the train than you might think.

One thing it might be worth doing is looking on sites like Shiply, where you can put up a transport need and people can bid on it, because you might get lucky that there's also someone looking to do a shipment approximately south to north, and suddenly a cheaper rate both ways makes sense for someone with a van. The problem you might have here is getting a good deal on the exact date and time you want (because the cheapest rates come when they can align your trip and the one back up), AND tbh finding somewhere to have your shipment stored.

You might also want to consider just carrying as much as you can with you, and buying some stuff down there and leaving it there between visits, even if you pack it in those vacuum storage bags. It might actually work out a bit more expensive than moving it all down there, but you'll only buy it once and you won't have to bring it back with you...and it will make your life a lot easier, which is worth money on its own.

I know it's not really answering your question, but people move their entire lives around with them in bags when they need to. People 'over pack' for multi-week holidays with nothing more than one suitcase and a backpack.

Basically, you can absolutely be portable for a few week's worth of clothes and personal items if you really wanted to, and without much real compromise. I've travelled for work a month at a time before now and basically managed it with just a large duffel bag and a backpack, with some washing powder.