Looks like nobody knows. :-k
The light fittings you are looking at in Screwfix have IP65. Technically, this means they are totally protected against dust ingress and low pressure water from any direction (but limited ingress permitted). Should be ok from that point of view. Of course, any holes for wiring etc have should be made waterproof, too.
Flourescent lights are about 40% efficient (I think). If your total will be 120W then you will be generating about 50W of heat. This will mostly be in the ballast part of the light fitting (if that's how these ones work - can't see). Spread over 1500mm of metal frame, that's enough to make it warm but not too hot to touch. Most of the heat will rise - I really do not think you will cook the plants below the light with this arrangement.
You could hang them an inch or so under the bench above them using a hook and chain both ends (that way you can easily take links up or let links out to raise and lower) - probably safer than screwing direct as it allows the air to move - or put something tropical above them! Probably worth putting tough plastic sheeting on the bench above them, too, so no water falls on them, just in case.
Be careful that any electrics are reasonably sealed, and environmentally ok if you are in a high humidity environment. Silicone sealant is usually good for sealing all the gaps (apply when dry and leave for 24hrs).
Can't say if that's enough light or not - hopefully, someone else can answer that. I know the type of light is important (warm white?) and extended light hours are usually necessary. 1-2' may be too far, but then you are supplimenting so not sure. You can always move them closer or further away depending on what your plants tell you.
I hope to have a greenhouse soon - hope it can looks as good as yours!