I always think it some what ironic that trickle vents are considered by building regs to be 'energy neutral' -which to me seems a convenient method of building control avoiding the conflict between part F and part L.
In the same vein, lockable window handles, good for part Q. security, less good for part B fire safety.
Trickle vents have their use in theory, although Ive yet to come across anybody that uses them at all. Customers generally hate them, especially on traditional period windows.
My experience of building inspectors is that they will glance at trickle vents and check there is one one a window in each room, Ive never seen one check the number square mm's of ventilation is sufficent. I also know its not uncommon to screw on a trickle vent with no hole behind and remove after inspection.
The advise I have been given regarding changing existing windows, is that its best to take a photo as a record then that will suffice to prove the same level of trickle ventilation has been maintained. Similarly with fire egress. Fire egress is difficult, its very easy to alter the size or configuration of a window and make it non compliant. A good example is a sliding box sash window that has been changed for a plastic casement, often a bottom half fixed sash and the top half top hung, it makes the opening part above 1100mm above bedroom floor.