I'm not an expert on Charity law, but the minimum criteria required by any organisation to retain their charity and tax-free status (as defined on the UK government's own website) is most certainly not being met by the WTBTS/JW's.
The PDF flowchart on this page of the web site should convince most thinking people that the WTBTS is not acting for the public's best interests:
The WTBTS/JW "charity" is a self-serving entity whose sole purpose is to perpetuate its own existence via the recruitment process of new members by existing unpaid members.