I think there is a requirement for trickle ventilation but the source doesn't need to be the windows*, it just happens that this is normally the most convenient place for it.
Is this a supply only job, a supply & fit by fensa approved, a job already involving building control?
If it is a fensa job then tell them that the ventilation will be provided by other means. In the very unlikely event that the fitters notice then tell them that there are wall vents going in later. If you have a BCO involved then you will have to do something but will be able to put in your own pretty ones or may be able to provide an alternate source of ventilation, ask him/her.
Sorry I don't have a reference to support this belief so please do your own homework to support it.
Why make the assumption that they will fit "horrid plastic things"?
If you discuss your requirements with the fabricators they can probably supply a vent that you like. Fitting a DIY set after installation could be a messy chore.
In certain circumstances you can avoid the need for trickle vents if you have lockable top vents with a "night ventilation" option. Fred has also pointed out that passive ventilation does not have to be through the new window frames.
Without details of your order and the source of supply (which I don't want! :)) it is not possible to give specific advice other than to suggest that you tell the suppier what you want.
If you spec windows with two stage locks that allow them to be "closed" but still slightly ajar then you won't need separate vents.
(I know this, because I speced trickel vents on the windows, showed them to the BCO along with the two stage opening, and he said that they would have been happy with the openers alone!)
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