I think potentially done right it would at the very least double them with the possibility to get much higher.

This is not even vaguely within the realm of possibility. Netflix has carried hugely popular shows before and their best efforts haven't even come remotely close to doubling their user base. You're forgetting that it's not just a question of Netflix users, it's a question of international availability. Netflix is not available everywhere and its selection differs country to country, this is because it is a privately owned, publicly traded entity that has to negotiate through more general legalese and rights management with a wide assortment of other entities both public and private. The BBC still negotiates broadcast rights, but the way they are setup as an entity makes it profoundly easier to spread to new territories, sometimes with compromise(notably the BBC America edits that use different music and shave the runtime down to ~49 minutes instead of the full 60).

I don't think you appreciate just how difficult and complex it is to double your subscriber base when you're already at the level Netflix is at. If they became the sole proprietor's of Clarkson, Hammond and May, they'd attract best case maybe another couple million viewers, absolute best case. And these would be the viewers who are more fans of the trio than cars and the TG brand/show overall.

Don't be so quick to buy into the "350m viewers worldwide!" figure, is the point, while that may be strictly speaking, accurate to some measurement, it's in no way indicative of how many viewers would actually follow them or more importantly how many even could