Also why are these politicians raising their voices at this point of time? What were they doing in the last 10-20 years?
Generally speaking, politicians stand to gain by adopting the policies of their constituents. About half of Britain's population supports Brexit currently, while not nearly as many pushed for it 10~20 years ago, which explains why certain politicians are only speaking up now - there was just not a demand for politicians to do so earlier.
TL;DR: politicians are not raising their voices "only now". The referendum is causing all sides to speak up.
Euroscepticism is not new in the UK. Factions of most of the major parties have been skeptical of British involvement with Europe. Some, such as the more nationalistic minority groups have been quite vocal on this, often along fairly racist lines.
In fact, the UK has a party (UKIP) dedicated to exiting the UK from the EU. The conservatives have seen some high profile defections to UKIP over the years.
There has been commentary that the current referendum was more about internal party control than popular will. Although, it is now quite hard to tell at the moment what the internal state of parties are.
Politicians are being more vocal now in order to promote their position on the referendum. The more strongly Eurosceptic the politician the more likely they are to start banging that particular drum, so to speak.
You would see this same effect on any divisive topic.