Well first of all you are already on the property ladder and as such a major part of your wealth is invested in the property market. To add a second property would mean that you are concentrating your financial risk rather than following best practices and diversifying those risks. And borrowing to achieve it just adds to that risk!
Given the current economic situation, the high level of borrowings and the excessive housing stock, no one can tell how this will turn out, so why exposé yourself to these risks unless you have to? In your current situation you will benefit from rising prices, while suffering little from downward trends. Add a second house with it's associated borrowings and you have have a very different picture.
My bottom line is to never take on more financial risk than you absolutely need to.