This kind of thing (an implicit clearance through CAS) is quite common, though more common abroad (where transits are generally much easier to get) than in the UK. You can fly all the way through France for example without ever getting one "clearance" through Class D - they aren't bothered.
It's best to always call them up with "G-XXXX confirm cleared through CAS on route XXX-YYY". That puts the ball in their court.
If you are on a radar service then it hardly matters though.