To use the tousantcovid, you need your nhs 2d code, I got this via my NHS login and from there get a pdf print out.
Use the french app to scan the code and it should recognise it.
You will have two NHS bar codes, it is the second one that really matters as confirms that you are fully vaccinated.
For the return, i have just ordered a couple of lateral flow tests to use.
Edit, The NHS codes are now fully functional in France, but not in all EU+ countries. ie French app ok, Swiss not ok.