Going by the 2005 church census, attendance in various denominations in England was as follows, with JWs included for comparison:
Jehovah's Witnesses: 100,000 (estimate assuming slightly lower attendance than publisher number and excluding Scotland and Wales)
United Reformed: 69,900
Salvation Army: 47,600
Seventh Day Adventists: 16,800
Although Baptists and Pentecostals are counted together they actually comprise lots of smaller organisations of a few tens of thousands of adherents each. Plus there are a dozen or so independent and new churches ranging from 1000 to 45,000, none of them equal in size to JWs. Mormon attendance is a mystery but assuming a generous "activity rate" of 40% would not be more than 70,000.
Including world faiths, JWs probably rank in the top ten religious institutions in the UK in terms of active members.