Just quickly from off top of my head, I believe they shifted some aspects of control onto the 'Christian Congregation' corporate entity, as this is not registered as a charity in the UK. That came up in the First Tier Tribunal Hearing. Circuit Overseers report to that according to evidence given there.
To the best of my knowledge, 'The Watchtower and Bible Tract Society of Pennsylvania - British Branch' was dissolved on Wednesday, 11th October 2000. See Charity Commission site: http://beta.charitycommission.gov.uk/charity-details/?subid=0®id=241623
The Watch Tower Bible And Tract Society of Britain had been incorporated nearly a year to the day before that.
Currently:
"The charity, a UK company limited by guarantee, is controlled by its governing document, the memorandum and
articles of the company dated 25th October 1999, as amended by special resolutions dated 19th April 2000 and 21st
November 2013." (Annual report, 2014)
Can't link directly because it's monstrously long link, but you want 26th November 2013 on the documents available (free of charge) from Company House: https://beta.companieshouse.gov.uk/company/03858051/filing-history
Hope that's of use.