Low E glass would be pointless, you'll have it all on keeping a sunroom cool, you don't want to start warming it up.
Secondly regs state if its not a habitable room then it does not require low E
Whats regs do state is that any glazing that is less than 800mm from the floor and any glazing that is within 300mm of a door handle ie a side panel has to be safety glass. The two types of glass that would be suitable for this job are 'toughened' and 'laminated'.
For single glazing then laminated would be the better choice, 6.4mm would be the perfect glass for the job, available in clear, stipolite and sycamore patterns. Thicker is available but is generally used for shop fronts
One draw back of lam though is its heavy, on the plus side its very resistent to burglars, can be cut from stock
Toughened glass on the other hand starts out in life as normal float glass, the weakest type of glass, it is cut to size while still in this form, it is then passed through a heat treatment process together with a bit of magic and with a rapid cooling at the end produces toughened glass.
Because this glass started as float you'll have various thicknesses to choose from, 4mm, 6mm, 8mm, 10mm right up to some stuff you can't pick up without a crane.
Draw back of toughened glass is that it has a mind of its own and can break whenever it feels like it and if it does break then it shatters in to millions of small fragments (hence safety glass), these wont cut your arm off but just leave a few pin pricks, it will also leave a large gapping hole that needs boarding up, hence why is no longer used as shopfront glass.
As for double glazed units then generally you would use toughened, its lighter than lam and if your governed by your rebate toughened is thinner, but it is not uncommon for home owners to mix n match both types of glass within one unit, laminated being used for the outer pane for security reasons.
7mm double glazing, not sure where or how you came about this so can't comment.
All glazing is measured width first then height, don't be caught out, if your ordering glass make sure you write the width down first then the height, also give the sizes in that order too, the glass shop won't ask, they'll assume you know what your doing, on a clear bit of glass it wont matter but on patterened glass you'll end up with the pattern running horizontally not vertically.
Thickness of dg units are given like this..... 4-20-4, this signifies a 28mm thick overall unit using 4mm glass, 20mm cavity, 4mm glass, theres a whole host of cavity sizes you can have starting at 6mm and going up to 20mm, in 2mm increments.
I know i've gone on a bit but im sure you'll have all the info you'll need now, if not just ask.
