Easy one, chap. Nip to yer local bike accessory shop and grab some anti-fog spray. There are several brands, costing about £4 for a little bottle.
Clean and dry the inside of your visor, then spray a good few squirts of this stuff on the inside. Rub it in until you get as streak-free a finish as possible.
Then go ride. Simple as - Job done. One coat lasts 2-4 weeks on average, assuming you ride for at least 2 hours every day.
I bought my bottle from Hein Gericke about 4 or 5 years ago and I'm still using it!!
I know there's a ProBike in Newbury and several places, including a Hein Gericke, here in Reading. Any difficulties finding it, gimme a shout, OK?