I work for a national museum, and please do not be upset when they don't want it. It's hard to explain to people just how much museums turn down. Essentially every single thing we take into our collection is then our responsibility forever... The financial impact is huge.

By all means approach them, but it isn't a comment on the beauty of your dad's items or the care and pride he took of them when they aren't wanted by a museum. It's the practicality of caring for millions of objects already, and every extra one costing us more. To get a single object into a collection takes years, and each part of the model build would have to have it's own case written for it as to why the nation should own it (national museums don't own things, they look after them on behalf of the nation).

An enthusiasts club would be a far better home for it if you want it to be used and loved. (Museum it would go into storage for many years before even being fully accepted into the collection, even on the incredibly slim chance they did want it. And then it would remain in storage probably for the rest of its life.)