To be clear, tax evasion is illegal but tax avoidance is lawful. In other words, organising your financial affairs to reduce the amount of tax you pay is perfectly legal.
There has been a lot of correspondence recently on the landlord forum Property118 about the legality of certain schemes and ‘Landlord Today’ has just reported that HMRC is not happy with ‘hybrid schemes’ that involve limited companies and LLPs (limited liability partnerships). Just today, the NRLA has suspended its partnership with Less Tax 4 Landlords pending further clarification. You can draw your own conclusions but HMRC has suggested that people that have employed hybrid schemes would be well advised to contact them to help unravel the mess they may now find themselves in. Useful links are below.