Since I posted the 200k membership another 1000 have joined. If it carries on at this pace by summer >300k membership is possible
But you are failing to grasp the difference between a political party and a business.
Anyone can join Reform. Anyone, from anywhere in the world. Anyone who wants to give them money.
If they were a constituted political party they would be bound by the rules of political party membership. They would need a constitution. Only UK citizens could become members. Members would not be allowed to be members of other political parties etc.
As it is, you can be a member of the Conservative party but also a "member" of Reform. You can be a US Republican and be a member of Reform in order to give them money.
The 200,000 number represents the number of people in the world stupid enough to give them money. They have a huge lack of talent pool to exploit.