There is a widely held view that UKIP supporters are disproportionately former Conservatives, particularly in the south.
Whilst this may have been true four years ago I don't believe this is not the case now, and in the Midlands and the North UKIP probably damage Labour.
The following table, however, is of interest. On 22 May we had a district election and a town council by-election in the same ward, with identical boundaries. UKIP contested the district seat but not the town council seat. Here is the result
Food for thought for those who think swinging to the hard right will 'bring home' the lost UKIP voters.