Agreed, it is technically an offence but as long as you see the TM first, they won't have a problem. In fact, the Avanti Train Manager's tend to stand next to the barriers at London Euston to assist in this very thing. Quite common to have at least one passenger wishing to buy a ticket at the barriers, but at non-barrier stations like Liverpool Lime Street, they tend to seek out the TM first and then buy a ticket en-route to Runcorn. Should buy a ticket beforehand yes, but as long as you're not avoiding the TM, you won't have an issue.