The rent is taxable income, but there are lots of things you can offset against it to minimise the tax. The main one is interest payments on any loan you take out to purchase an investment property. I was advised by my accountant that providing it is done transparently, if you fund a purchase by extending your own mortgage, then the extra repayments would be allowable against tax... by transparency I mean that if you buy a £150K house on 1 February, and your mortgage statement shows a further advance for £150K was made on 30 January, then that's pretty transparent! I think it's much better to get a separate mortgage for the property though; no risk of the claim being rejected then. 
