I've just ordered a cheapo chinesium suppressor from Aliexpress and it's gone through heathrow no problems, just waiting for it to get to my local delivery office.
I know a lot of it will depend on how the package is labelled and what it looks like (ie if it says "gun parts" then a red flag would be raised. I think the majority of those sellers are wise to this and try and make it being stopped as unlikely as possible.