So, this might glitch and fail, but here's what (appears to have) worked for me.
OEM SD card: 16gb (but less, really, something like 14.8gb capacity)
Full European maps download: 17.2gb
Needed more countries than any one of the bundles included, and as far as I can see, you can't combine bundles.
1. Downloaded full maps (link above)
2. Extracted the 7z file.
3. Deleted all the non-English language folders from Maps -> 00 -> sds. Keep GB folder only, and the meta_info.db of course.
4. Deleted all the files for other VAG brands from maps -> 00 -> cfg -> mapcfg and res -> brands.
Still at 16.2gb...
4. Identified the folders (regions) in the nds folder that I would not be visiting. Copied and pasted all the files from the smallest region (E13) overtop the files in those folders.
AI search says...
E1: Spain, Portugal
E2: France, Belgium, Netherlands
E3: United Kingdom, Ireland, Iceland
E4: Germany
E5: Denmark, Finland, Norway, Sweden
E6: Croatia, Czech Republic, Hungary, Poland, Slovakia, Slovenia
E8: Ukraine, Belarus
E10: Albania, Bosnia & Herzegovina, Greece, Kosovo, North Macedonia, Serbia
E11: Austria, Italy, Switzerland, Liechtenstein, San Marino
E12: Luxembourg
E13: Estonia, Latvia, Lithuania, Moldova
5. I was now at 13.8gb! Backed up, formatted and then copied the files over to my SD card.
6. Spot tested in the van. All seems fine. I can find and navigate to my intended countries. Searching for those I overwrote... they don't come up.
I think this works because I've just replaced the data that was in those locations with data that is still of the expected format. If you just delete the data, the software is unhappy that it is instructed to look there, and the files are missing.
My advice?
Just buy the 32GB card from @Pauly for £25. See his post here.
Or, if you're like me and don't have time, and like to solve problems in order to save even small amount of quids, try this
It's a backup for my CarPlay maps... so we'll see how it goes!