They certainly should be, though be aware that Chiltern's booking engine does a £1 pre-authorisation before requesting the authorisation for the full price, which can occasionally trip card issuer anti-fraud measures.
Practically on the second you receive the code, you can likely collect that.
Sometime I just stand near the TVM and use cashback site to buy the ticket, and insert the card to TVM while the final transaction is taking place.
I've collected within seconds many times. The £1.00 charges are annoying as they take 7-10 days to reverse - they used to pend for the entire time but now go through as charges before reversing days later.
I use topcashback / quidco to buy and am always able to select the next train that's due within 2-3 minutes of departure on the website.