That's a slightly tricky minefield of a question. I'm assuming you mean a USB port on a telly that takes a memory stick, mounts the filesystem, and plays AVI/WMP/MPG/MOV/etc.. file types (and probably displays images and plays music and other whatnot).
A legal way that I can think of is a PVR computer. You can buy/build a computer with a digital freeview/freesat decoder in it, and there is software out there that will let you schedule recordings based on the available program information that is also broadcast on the digital airwaves along with the programs themselves. You could then copy to mem-stick and shove it into the telly. A better setup might be to have the PVR computer connect directly to the telly, but what the hell. This is entirely legal (if you have a TV license) as it's equivalent to home taping of a broadcast program.
iPlayer can/does give you the ability to download the programs for later use, but I'm not sure if the codecs have been released to telly manufacturers to allow them to decode it. I've heard of programs that can take the downloaded file and strip the DRM off it and convert to a more portable format, but have no experience of that to comment (and it is possibly illegal in countries that have a DMCA equivalent piece of legislation). There are now tellies that have an inbuilt connector to allow you to connect it to the internet (usually an RJ45 port, haven't seen a wifi connection yet) and they can access the iPlayer, but this is different to what you ask about.