I have never been convinced that 'second homes' a major issue in the UK (although they might be important for a small number of rural areas). The Census figures would appear to confirm this - 165,095 people (approximately 0.3% of the population) report a second address that they use for more than 30 days of the year. Many more people report second addresses (1.57m) but the vast majority of these are students living away from home.
Even the 0.3% figure is likely to be an overestimate of the number of holiday homes. According to the BBC: The Office for National Statistics said the estimates of the number of
people with second addresses used for holidays were "not equivalent to
an estimate of holiday homes". Although, somewhat oddly, that doesn't seem to have stopped the BBC from providing a map labelled 'holiday homes mapped' which appears to be based on the census data.