I agree with Van-Wild.
The MTP smock that I have seen doesn't look like something you would put under a windproof. It looks like an outer layer.
I have used a very light shell by Arktis, their "Rain Shield", which is very packable and meant to wear under or over. When worn under a cotton 95 jacket the jacket just got soaked and acted like a wearable refrigerator, when worn under a Swandri jacket it stopped the wind alright, but I got no warmth benefit from the Swandri. Worked much better with the water/windproof on the outside.
I have two of the Paramo light weight Velez smocks, an old style large, and a newer medium. I tend to use the large and put on and take off, as part of a layer system that includes a fleece and Paramo Fuera windproof smock. For me, the Velez is too warm if I am moving, even over a short sleeve T-shirt, in anything other than the middle of the SE English winter. I still like it, just wish it was a jacket with a full zip for better venting.
Speaking of different philosophies, I think the Gore-tex jackets by Outdoor Research have a different one compared to the military kit. I managed to get a medium OR Foray jacket at a great mark down. It fits me, arms are the right length, body is right length, but it is a fairly slim "active" fitting, not meant to go over lots of layers. The arm holes get tight. I got the Foray as a summer packable waterproof, for times when I found the Velez too warm, and bulkier than I wanted to carry just on the off-chance of rain.
I have never thought a Poncho would suit the stuff I do when I want a waterproof, but for walking it could be a good choice, and certainly cheaper than the stuff I have bought over the years. Could also keep a pack dry, limit damp shoulders under pack straps. Not sure how ponchos work in driving rain, which I would imagine does happen around Dartmoor.