You will have a problem with bad flow to your shower head at its highest point in that case. The problem you have, is the height of your head of water. You will get a flow of water through your shower head if you hold it down at bath tap height, but as you raise it up to a level to be able to...
Do you have equal pressures on your bath taps?
Ie: Cold and hot fed indirectly via a cold water storage cistern.
Equal pressures are a must for thermostatic showers. For non thermostatic, not so much so, but a lot easier to blend if you have equal pressures.
Make sure the flush pipe is entering nice and square, that can also make a difference to a slight leak.
The one shown in the link above is normally a good one. You shouldn't have to muck about with all that silicone malarky, it looks messy. Especially round a chrome pipe.
Good luck.
Tell us a bit more about what plumbing system you have. That would help.
Why would you be running a bath, and having a shower at the same time, anyway?
Looks to me as if that should just come off, give a a bit welly. Or try to lever it off in different places with a flat head screwdriver between the body of the tap, and the head. Is it a thermostatic Aqualisa bath shower mixer? Looks like it to me from that picture, I could be wrong.
It's one of those things. You'll find some plumbers do, some plumbers don't. Either way really.
Just don't over do it with silicone if you do, because sometimes it can make the rubber washer slip out of place when tightening.
As has already been said, it is very amateurish. You are well within your rights to get him back to sort it out. Don't let him fob you off about an unlevel floor, that's no reason for him to install the tray unlevel. This is a very easy problem to overcome. If the tray dosent have legs (which...
If you don't fancy using a flexi pan connector, one of these should do the trick.
http://www.mcalpineplumbing.com/4-110mm-straight-adjustable-length-rigid-wc-connector.html