// Who out of those boat people - many of whom come from Africa - would come here if they knew they would end up in Rwanda?//
Again, see the data above, many *don't* come from Africa, so already your premise is flawed. Secondly, the word "knew" is doing a lot of heavy lifting there.
Firstly, the people coming have to *know* about the policy. Most of us don't know much, if anything, about how UK Migration policy works, so why should people elsewhere be so deeply informed? Many won't know, either because they didn't do proper research or were lied to, or some other reason.
Secondly, even if they knew about the policy, they might figure it to be worth the risk -- it only applies to people who get caught, and while as far as I know most people attempting to enter on small boats are picked up, migrants might take the risk anyhow (either by underestimating the chances of getting caught or by deciding that the small chance of success is enough to justify a crossing).
And thirdly, because if you have 50,000 people arriving in small boats or whatever and only send 200 to Rwanda, then even if arrivals knew about the policy and expected to be caught then around 49,800 of them a year would still end up being processed here. Rwanda has neither the capacity nor the will to take all 50,000+ arrivals here, and once they've reached their maximum tolerance then, lo and behold, whatever deterrent there was comes to an end.