The farther away you get this antenna from any metal, the better it will work. I've already had some experience with this exact type used in another project (ClockworkPi uConsole).

Its better in your case because it's not directly fixed to the metal but of course you'll always have shielding and reflections from nearby metal objects. In a compact device you have to live with that.

Measuring with a VNA (as mentioned below) will not give you a lot of info. It would tell you how good the antenna works generally (ideally measured as far from any metal as possible). In your enviriment, you already know that the situation is not ideal, so not a lot to gain from measuring it.

A fractal antenna design that could be used for Wifi, Bluetooth and more would be awesome. I have no Idea how to do this though and still consider it Magic

I think the easiest way forward would be to make the copper heatsink asymetricly smaller on the antenna side and to check if there is better (more RF transparent) PCB Material available. These specs should be available. There are certainly better ways, but this is easy and probably worth a try.

@minute @poleguy @morl0ck