Mark - I don't know anyone who has confirmed that they will have tetrataenia litters in the UK. There's a few who may have them. A good bet is to search on the RFUK classifieds, not for whavis available now, but for people who bred them last year - there's a fair chance they'll be breeding again this year.
I've not handled an infernalis, but I have handled a tetrataenia and she wasn't aggressive. She was nervous, but not aggressive. Certainly no more flighty than our Cuitzeos. It may be that the tetra I handled was calm for the species.

If you're looking forward good handling garter I'd recommend radix or marcianus. It depends what you want, if you want a stunningly good-looking snake a tetra fits the bill.