The water tight height is about 4" inches.
As long as the GRATE is not blocked, the GASes will find the easiest way to escape, that is to enter smoker and escape from exhaust stack, other than to overcome the pressure of 4" height water pressure to esacpe from cover of fuel bin.
In my smoker I do not use an exhaust stack, there is no "SUCKING" function. The gases still escapes from gaps of cover of smoker, it is easier than to overcome 4"water height pressure.
If you use a STRONG BLOWER, it you BLOCK the grate, the positive pressure created inside the burner may overcome water pressure of 4" deep water around the cover.
Possible reasons of UK problem:
#1. Low quality of wood pellet, possible with high content of sugar, adding too much mud(earth), or add cornstrach, or other additives. Theoritically food class pellet must be pure wood.
#2. too strong blower
#3. too small grate gaps.
These 3 points are just my guess. I never see the real burner, I never see a photo, I never get any data.
There are several gravity feed smokers in the market like Backwoods, Stumps, Viiking, Superior. All fuel bin covers use gasket to seal. I used gasket too. But some time later, the gasket will fail and you have to change new gasket. Water tight seal is more reliable.
Again if you worry explode in my burner, there shall be many expoldes already in Backwoods, Stumps, Viiking, Superior.