If the boss of a private firm, in the UK, has consensual sex with the 16 year old office junior it is not a sex offense ["offence" where I come from.]

Well,
It is fairly simple: if you have sex with someone who does not consent or who (for any reason) CANNOT consent in a clear-headed fashion, you're human offal.

And, by implication,
In an employer-employee relationship there is a power imbalance that I'd have thought would prevent the junior in the office from giving "informed consent" (as it's called in other contexts). If the consent is not "informed" (and thus "proper") it's not really "consent" and the sex is not consensual.I

In which case,
There is an argument that, should the boss of a private firm, in the UK, have sex (with a flawed definition of "consensual") with the 16 year old office junior it should be categorised as a sex offence - and s/he should be prosecuted - and have their name added to the sex offenders register.

Cheers, Rowan