I invest in Greater London because of the capital value and because that's the market I'm most familiar with, but moreover, Greater London and the South East is where the best tenants are. Finding the right tenant is crucial, no more what sort of property you are renting.
Aside from tenants who aren't ready to buy because they're students or moving around for work, where property prices are low enough to enable a single person on a modest salary to buy, you are going to find that most people who rent are not in steady jobs, and are on low incomes and housing benefit. This means more risk of rent arrears and as such tenants have much fewer housing options, they are harder to evict should that become necessary, especially if they're reliant on the social sector to rehouse them.