T&M did sell potato seeds a few years ago, but they've stopped now. Are you planning to use the seeds from your own spuds? It would be interesting, even if they aren't very productive, as you might just get a useful new variety.
Onion sets - I'd agree that if they aren't F1 hybrids, they are probably worth trying. The best allium to allow to go to seed is theleek. As well as seeds, they produce miniature leeks which you can plant out & get a flying start with.