I have some btl so will probably get a heap of abuse for this post, but I am completely confused by all the 'tax dodging' people on here seem to think happens. If you have btl's, you are expected to complete a full tax form and keep accounts - it is a form available with the self assessment tax returns.
Most landlords only have a couple of flats. Lets say they bought a flat for £100000, 75% loan @ 6% and get £500pcm rent for furnished flat.
Income: £6000
Less Interest: £4500
Less 10% Wear/Tear £600
Less Insurance £150
Total: £750 taxable income
The real tax liability comes upon selling the property (assuming that there has been a capital gain!), and the government can track these thro the stamp duty forms which are now completed even if the property comes under the stamp duty threshhold.
The point I am making is that even if the rent is not declared, the likelyhood of there being an enormous tax bill at the end of the day is pretty low for the majority of landlords.
Btw - you are right about Scotland having mandatory registering for landlords, but it is pointless - all you have to do is tick a box to say that your propery meets legislation and then pay £11 per property which registers it for 3 years. The only thing they do check is whether you are a 'fit and proper' person to be a landlord. !?!
D