Edit: It's so low because I buy properties that are in good shape. I have a full time job so I wouldn't have time to focus on doing repairs and improvements.
More specific ROI calculations are good, but people typically forget a lot of expenses (repairs, vacancy, time spent managing, etc) and end up calculating an unrealistic ROI. After accounting for all that stuff, at least 8% cash-on-cash, plus more in debt repayment.
Actually, it depends very much on your circumstances, attitude to risk, how much you're happy to pay in agency fees etc.
Also, in the UK, we don't tend to have 30 year mortgages, which makes predicting a stable price very tricky.
The UK also has vastly different tax rates - and a changing rental culture - which may also affect the price you want to charge.
I am completely ignorant about UK real estate, so I might be missing something obvious.