Double check there is no shorting between the signal wire and the braid both on the aerial feed cable and the patch cable.
I am reluctant to go to the shop and buy a more expensive aerial if the underlying issue is a box that essentially produces so much interference that it causes loss of signal.
Have you enabled RF LOOP THROUGH in the YouView box's settings menu?
Press the blue Y button, scroll to DEVICE MANAGEMENT and make sure it's on. If it's not, it won't share the signal with the TV.
But the YouView box works fine? You can watch the aerial channels OK via that?
No.. as soon as youview box goes on, signal dies. I have to do a merry dance with the aerial to find signal. I find that in order for the aerial to find signal it cannot be in direct line of sight of the youview box, which again points to interference
It's an indoor aerial then? Is it possible to borrow one from someone? Or to buy a lowcost one to check what happens? Does your current one have some kind of booster? Is it a powered model?