They were but DCLG retracted the requirement last month. It now is
"3.4 Where the original windows were fitted with trickle ventilators the replacement windows should include them and they should be sized as set-out in paragraph 3.6. It would be good practice to fit trickle ventilators (or equivalent), with accessible controls, in all replacement windows to help with control of condensation and improve indoor air quality."
SOMETHING is required by building control. It doesn't HAVE to be trickle vents - they are just one way of achieving it.
But we're talking about replacement windows. Incorporating a trickle vent is an easy thing to do at that stage. Adding airbricks, etc, isn't.
It will have a means of locking it open about 1/4 inch. That serves the purposeas far as BR is concerned.
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