Sounds to me like your expectations of a cheap $30 knife are just too much.
Smacking the back of any locking knife is generally considered abuse and just about all will fail from it. I remember reading a thread where someone else had this expectation and even $50-100 Spydercos and Benchmades failed the test. Folding knives just aren't made to do this.
Randomly stabbing things with any knife, much less a folder, is abusive as well. A wood fence makes even less sense. Just because the surface has rot on it doesn't mean there isn't hard wood underneath. And because it is a fence you also have nails/screws/hardware to worry about. Sounds to me like you got a new knife and were a little overjoyed so you started to look for reasons to use it. We've all been there. Next time, try to be lest stabby.
Obviously pictures can't really do justice when it comes to blade play but given your previous expectations of use of the knife I would again have to say it is only a $30 knife. If you want something that locks up tight, cand stab anything, has an edge that lasts for a long time, and can be smacked around like a fixed blade you are going to have to spend a heck of a lot more than that.
Side note. Aren't locking knives prohibitted in the UK in some way? I remember seeing some Spydercos on closeout somewhere that were UK versions with no lock and my understanding was that was a requirement there.