"According to the OEC, the UK is the 10th largest exporter in the World, but only 21st by population. I'd say that the UK is "punching above its weight"."
Quoting a number or two in isolation may seem to bolster a flawed argument, but is close to meaningless.
If we look at a list of per capita exports, the metric you chose to use, we find the following countries AHEAD of the UK, with the first twenty-five producing double the UK value for per capita exports:
San Marino, Lichenstein, Singapore, Hong Kong, Qatar, Switzerland, UAE, Kuwait, Norway, Netherlands, Brunei, Luxembourg, Belgium, Iceland, Monaco, Equatorial Guinea, Sweden, Denmark, Puerto Rico, Germany, Faroe Islands, Iceland, Oman, Bahrain, Slovakia, Finland, Saudi Arabia, Aruba, Slovenia, Canada, Taiwan, Czech Republic, Australia, South Korea, Estonia, Malta, Trinidad and Tobago, Lithuania, Hungary, France, New Zealand, Libya. Israel, Italy, Malaysia, American Samoa
Then comes the United Kingdom in 47th spot, just ahead of Greenland, Spain, and Gabon.
Other versions of the list were similar, with the UK hovering just above 50th in the world by this measure. They might just squeak into the top quartile if you can get enough remote Pacific micro-nations on the list.
I expect that the UK 'punching above its weight' in economic terms is yet another of their local cultural memes that is not supported in the real world.