Postal delivery staff are still instructed to scribble across any (apparently) unfranked stamps that they spot. Such an instruction would be unnecessary if those stamps had already been invisibly franked. Further, I can't see the Post Office unions agreeing to the use of invisible franking. If it existed, stamps which were reused (because there was no visible franking) would be rejected by the sorting system, resulting in postmen being required to collect excess postage from the recipients of letters that had apparently got valid stamps on them. (That would inevitably lead to lots of doorstep arguments and possible assaults upon postal workers).
Since this question was posted under 'Law' it's worth mentioning that knowingly using used stamps to obtain free postage is a criminal offence under the Fraud Act 2006. It's likely that persons selling such stamps could also be convicted under that Act but only if it could be shown that they knew that it was likely that the stamps would be used for fraudulent purposes (rather than, say, added to a philatelic collection).
Chris