The concept of a national park is very different in the UK from that in the US, despite the concept being devised by a Scot, John Muir. In the UK the land, apart from a few exceptions such as military areas, is all privately owned. In the Cairngorms the park is owned by a large number of different estates, which include both the King as a private individual (Balmoral) and the National Trust for Scotland, NTS (Mar Lodge). https://cairngorms.co.uk/working-together/land-management/estates-map/ Apart from the NTS, all owners run their estates as a business, but apart from the very obvious grouse shooting, deer stalking, forestry and sheep raising which significantly affect the landscape, tourism is also a very big business. The NTS estate is very much more run along the principles established by John Muir. A national park is a high level conservation area, meaning owners are highly restricted in how they can impact the area, but it is not an outdoor museum which is preserved in formaldehyde. Some estates such as Glen Feshie are very focused on conservation and rewilding.

From the point of view of a backpacker, in Scotland, your access rights in a national park are exactly the same as anywhere else. The one exception, as Louise says, being parts of the Loch Lomond and Trossachs NP where special local restrictions on wild camping had to be introduced due to intense pressure and damage caused by the proximity to the densely populated central belt. Basically crowds of drunken yobs were trashing the place.

In the Cairngorms you are much more likely to end up having a chat with a friendly gamekeeper who drives up in a battered green land-rover who is very interested in what you are doing and who gives you useful advice about that nice little wild camping pitch hidden round a corner behind the trees 1 km further on. That has happened so many times to me.

Welcome, I've spent many a night hiking in the National Parks (US). I understand and appreciate the permit system, though somewhat frustrating in a last minute escape to mountains if your particular objective is subject to out of stock permits! You will love the 'freedom' that you will find here. Please do venture away from well trodden tracks and paths ( google earth will help you decide what is and what isn't well trodden), assuming your nav skills support featureless navigation. There is a real joy of navigating across pathless terrain, something I rarely had the chance to do in the US Parks since off track ventures can be really hard going (Deserts excepted!)
One aspect that I did appreciate in the system in high altitude fragile habitats was the 'pack everything , yes everything!' out mandate , with the odd ranger around checking that indeed your are carrying 'everything' out.

Andy

The only thing you’ll need to watch in your planning is that some of the Cairngorm passes (lairig Ghru - definitely and Lairaig an loigh - depending on how you access it) may need a FWA and all the summits will. The routes round the edge of the plateau are brilliant and well trodden by Challengers and don’t need FWAs.

Thanks all for your kind and very informative responses! And yes, I'll be sure to include any FWAs where necessary!