My MTB weighs in at around 14kg all in with knobblies and I'm currently using it for my 2.5mi each way commute and for site to site transfers - it's great in trousers and a shirt as it's a bit more comfortable than the road bike.
My "commuting" bike is a road bike with 28mm tyres which with lights, mudguards and all the other bits and bobs comes to around 12kg, it's much much quicker to get around on and much easier for distance commuting- in my last job I commuted 13mi each way and often ended up doing around 32mi daily so not dissimilar to you.
Whilst getting slicks on your MTB will certainly make the trip much more pleasureable and easier, I can't recommend getting a bike better suited to commuting enough - something with full mudguards, panniers and so on will make you faster and keep you fresher.
FWIW, the Triban bikes come highly recommended and as far as I'm aware do let you fit racks for carrying your stuff, although I personally prefer a rucksack.