I'm Canadian, but hey, that's still in the Commonwealth!
'freezy day' would be a colloquial phrase; not quite proper. In fact I dont think the adverb 'freezy' even exists, but there is a N. American noun 'freezie' referring I think to a juice popsicle or frozen drink. That's possibly a trade name to get around the trademark of 'slushie' by 7-11. However, when I imagine a cockney accent with the phrase 'what a freezy day!' it seems to fit. :)
Forecast is used all the time, and not just for weather.
'The company forecasted a 7% growth in the third quarter.'
'The forecast said there was a good chance of rain tomorrow.'
'Our best forecast says the Iraq war will be over in less than 3 months.'
'Are you going to try to forecast the winner of the tournament?'