In the real world most property investors aim to get the best return on their investments. Many of the best investors take a flexible approach: holding some property for long-term rental and selling other properties on in order to achieve better returns, improve cashflow, or to release funds for other investments.
In a sensible world, all these activities would be regarded as a single business. The investor is merely trying to achieve the best results with the assets and funds available.
For tax purposes, however, long-term investments and short-term developments have to be regarded as two different businesses - and the tax treatment of these businesses is very different!
When properties are held as long-term investments, they are regarded as capital assets. For individual investors, the rental profits are subject to Income Tax but any profit on disposal of the property is regarded as a capital gain and is subject to much lower rates of tax - generally 18% or 28%, with an exemption for the first £10,900.