Genuine Rivnuts are still made by BF Goodrich in the USA & until the mid 90's by Linread in the UK. There are many copies around. originally they were Flange & Csk headed. Avdel introduced the Thinsheet Nutsert (which has a very small csk head & falls through the hole unless it accurately punched/drilled, but works in thin sheet material(!)& gives an almost flush finish without countersinking)These have been widely copied & are made by every far eastern man & their dogs with quality from awful to excellent. Mostly available Metric in Europe & UNC/UNF in the USA.
Look for them as "Rivet Nuts, Blind sheet nuts, Thin sheet blind inserts ....lots of variants. Thin sheet inserts have very thin walls & can be installed relatively easily with hand plier type tools. Rivnuts are much more heavy duty & are hard work to pull up by hand. They can be put in with a cap screw and a couple of nuts & a washer to "jack the thread up. In the 60's Landrover winches were held to the chassis as a kit using Aluminium 3/8 UNC Rivnuts and an illustration of how to do it with this method! More tricky with smaller diameters.
In the 70's Linread made over half a million aluminium Rivnuts a week! UNC/UNF/BSF/BA & even lots of Metric for Johnny Foreigner!

Location: Stratford upon Avon

As a quick follow up Laser (who do induction heater kits!) also do Rivet nut kits, you should be safe with their eastern quality. Rivet Nut Co.uk may be worth a try as well

Location: Stratford upon Avon

Thanks again guys.

I have various sizes of Nutserts, including the tooling, which I have been using successfully for a while now.

I have been searching unsuccessfully for bsf ones and decided to ask here. I will try tapping the 6mm ones as suggested, I appreciate you all sharing your experience.

Some years ago the power ststions here were using British generators & electrical equipment which used all BSF nuts & bolts.
Somewhere in UK is probably a lot of scrap generators etc. could be worth trying to track down.

Location: AUSTRALIA