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It varies depending on the glass, but about 2.6 grams.
If you look strictly at the maximum weight of the trailer (2600kg) and the maximum towing weight of the Space Wagon (2700kg), you could technically do it. I wouldn't, though. Horseboxes have a high center of gravity because of the way horses are shaped, so you're going to need a better vehicle than a Space Wagon to pull this. A Nissan Pickup would be ideal. Hiluxes would not be--their towing capacity's too low.
My 1999 Multivan is plated at 2600kg (2.6 tonnes). I believe standard panel van T4 is plated at 2515kg (2.515 tonnes/ 2.47 tons). In any case, they're all over the 2.5 tonnes restriction for "larger vans" & "motor caravans" for the new Low Emission Zone (LEZ) rules from January, though you can get them downrated to 2.45t & re-plated, then apply to VOSA for new log book for around £120 + the cost of a VOSA weighbridge cert (£12-30) then registeredwith tfl for the new lowered & exempt weight. Cheaper than going into the zone & paying the £100 charge on 2 days !
It varies enormously. See the link below to a table showing all sorts of glass and glass-like products: 2.6g/cm3 2600Kg/m3 Also glass sinks because it has a density higher than 1.0g/cm^3.