I pity the poor fitter, who, in years to come, goes to remove one of your baths!
As for the "L" brackets, the problem with them is potentially splitting the wooden batten if you screw through without first pre-drilling. Also so many fitters (for ease of installation) put the bracket facing upwards (and then tiled over). Makes it a right pain if bath is to be removed without disturbing the tiles above!!
Why not put one bracket at each end (of the long side near each end) then one each on the near side, four brackets = bath reletively secure without the need for any adhesive, dont forget to use the lock bolts on the legs (they help stabilise as well and (before you tile) a sufficient but small amount of clear silicine between the bath edge and wall.
This silicone will be thin enough to secure but not enough so as to make it impossible to remove the bath without destroying the wall behind.
The finishing silicone then becomes a waterproofing and decorative layer and although it will also provide some mechanical grip that is not is what is intended, thus this top layer does not become stressed and fail causing a leak path.
www.iiplumbing-services-derby.co.uk