The WTBTS of Great Britain , or whatever its correct nomenclature, has been a registered charity for many years, in turn individual congregations were told to register as charities .
The advantage of this in the U.K is that all donations of money upon which any kind of tax has been paid ,i.e Income tax , are then recognised as non-taxable by the Government, and tax at the appropriate rate is re-claimable by the Soc. or Congo. So a 40% tax payer is really an advantage to a Congo, but there aren't that many of them, too many window washers, who officially don't earn much.(Its a cash business)
The other advantage is that funds going from one charity to another, Congo to Soc. etc. are not perused by the Tax Man, but come under the purview of the Charities Commission.
As to what "charitable" works they do , they will claim "Bible educational " work, plus teaching someone to read etc.
When I was first starting to fade, a guy on here, or one of the other sites, was trying to get the Charities Commission to investigate the whole scam, but it does not seem as if he had any success. The Devil (WT) is quite adept at appearing to be an angel of light.