It's quite interesting in Australia - Home mortgage interest is not deducted but Investment Property interest is considered a business expense and is thus deductible. (Which makes sense to a degree - purchasing an income-generating asset is deductible for any other business, why not in RE).
Same in the UK. However, there's a loophole, of sorts, that lets you claim a percentage of your mortgage interest in the UK if one room or area of the property is dedicated to your business.
