@Andrew3096 Be careful as I think you may be confusing two different things (but I am no expert on Welsh devolution).
For council tax/business rates in England you need to be available for 140 days and actually rented for 70. in Wales these figures are 252 and 182 respectively. Failure to achieve these figures allow Welsh councils to triple the council tax on second homes. Achieving this figures put you into a business rates regime and a lot of properties pay nothing as they get small business rates relief.
For income tax though I believe the rules are the same in England and Wales (but I could be wrong). If so then you need to be available for 210 days per year and actually let for 105 days. There are other rules like no booking can exceed 30 days but the 105 days rule is the one you need to achieve.
The test is done over the 12 months period from when you start letting and then over the period 6th April to 5th April.