Possibly out of date, but I did check with our BCO when I did our loft
conversion, and he would have been happy with the two position lock
instead of the trickle vent...

Yes, it's still the case that a locked-partially-open position is an
acceptable alternative to trickle vents.

However building regs do insist on some of permanent background
ventilation.

That can be a whole-house ventilation system, ventilation bricks, or
window trickle vents.

There's a formula for the vent minimum area based, based on floor area
of the room, but can't recall the % right now.