If you're busted for doing this, it will most likely be seen as falsifying your logbook, in which case you can kiss goodbye to any hopes of flying professionally in the future
What a lot of nonsense some people write.
Of course you can log the time.
You can write more or less anything in your logbook. A friend of mine logs time as passenger (his PPL has expired due to ill health) and he has hundreds of hours thus logged. You can even take photos of interesting places and paste them in your logbook!
However, the logged time will not count towards anything under the CAA or JAA regime. It will be just "passenger time" and basically you are logging it for your own satisfaction/record/etc.
It will be an illegal entry if you log it as "PIC" or "P1".