Polling evidence suggests that many former Conservative voters defected to UKIP in the recent European election. However, the party that fared worst in this election was evidently not the Conservatives but, rather, the Liberal Democrats. Based on the overall election results, my guess would be that: many former Conservative voters and some former Labour voters defected to UKIP, some former Lib Dem voters defected to the Conservatives, and a few former Lib Dem voters defected to Labour. (I would be very interested to know if anyone has any detailed evidence on this.)
The net impact of UKIP's rise on the Conservatives, at least in the European election, appears to have been negative. But there is at least one pathway through which UKIP's rise might have had, and might continue to have, a positive impact on the Conservatives. Many individuals who are liberal on social issues like gay marriage and abortion but take a centre-right position on the economy have traditionally voted for the Lib Dems. Despite being slightly closer to the Conservatives on economic policy, such individuals may have opted to vote Lib Dem for the reason Theresa May mentioned in 2002, namely that they did not wish to be associated with the "Nasty Party". However, now that UKIP is widely regarded as the "Nasty Party", and with socially liberal Conservatives such as David Cameron and George Osborne in prominent positions, many of these individuals may end up defecting to the Conservatives.