Decathlon have vittoria randonneur tyres for only £9.95 each. These tyres fit moutain bike 26 inch wheels and allow you to experience increased speed and grip when using your mtb on the road. I commute in London each day and these tyres even have a reflective strip to improve visibility! Just for comparison wiggle have these for £13.05 each.
The vittoria website says,
"Speed and safety on city roads in all conditions.
Thanks to its Aquaflow tread pattern, Randonneur is at home on the road even in the rain.
Randonneur features Aramid 3D Compound for optimal duration and grip. Double Shielding puncture protection and (optional) reflective bands."http://www.vittoria.com/index.php?option=com_content&task=view&id=8103&Itemid=122