I can't really recommend for a British firm, as the Dutch building styles / market is quite a bit different. We don't have any paid 3rd party libraries, but we do use a few of the things available on sites like bimcomponents (graphisoft depositort of objects) and bimobjects.com.
For your specific example:
The standard library should offer a good solution for sliding windows, as long as they're symmetrical, which I think is mostly the case in Britain. If you want to add fancy trim to it, you could either add a few 3d elements in your drawing to existing window, or you can save an entire 3d model as a custom library part for the window.
I allways try to get as close as possible with the standard library, before resorting to custom parts. Any adjustments made to custom objects take extra time.
There are 3rd party solutions to make custom doors and windows, but I have no experience with those.
If it is assymetrical:
Historical sliding windows in NL typically are assymetrical (the sliding bottom is bigger than the top). I generally 'fake' these by getting a window with upper part and making the sash that is inbetween as close to zero as I can. See below for a recent example that I'm working on.
Erwin Edel, Project Lead, Leloup Architecten
www.leloup.nlArchiCAD 9-26NED FULL
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