i am also in the uk and i use a 100 gallon flexi for my 10 month veiled.
I am not a fan of glass vivs and many people don't use them here because of the fluctuating temps.
However if you get higher wattage bulbs the basking temps can be acheived.
Mistings three times a day keep humidity fine and with an additional dripper i personally find is perfect.
I may be wrong and i'm sure i will get corrected if i am but,the chams i have seen in glass vivs over here always seem to want to get out for whatever reason