//…logically, why it would make no sense for an economic migrant to come to the UK rather than stop off earlier in their journey, at more generous countries that don't involve life-risking crossings//
They come here because, unlike most other European countries, they can claim reasonable benefits, have their medical and educational needs immediately fulfilled and can earn a decent living in the “black” economy. It's their "destination of choice." If they claim asylum at all, it is because, allegedly, they are fleeing danger and/or persecution. If that was the case they would be happy to claim asylum in the first safe country they arrive in (as A31 of the UN Convention on the treatment of refugees suggests). If, as you say, they are willing to face a perilous crossing and pay large sums for it, there is something in it for them over and above what they can secure in mainland Europe. The first thing that is in it for them is comfortable accommodation and immediate access to medical facilities if required. Far better than living in a tent in scrubland around Calais. If you still wonder why they head here, how many migrants do you see living in the woods under a piece of tarpaulin in the UK? How many do you see in France or Germany (see Khandro’s post). You've explained how much easier it would be for them to remain in Europe, so have a stab at explaining why they don't. It ain’t rocket science but if you think there are any other reasons do enlighten us.
//Couldn't we make our benefits and our healthcare slower to access, bringing them into line with other countries?//
No, because the handwringers (including many in the so-called Tory Party) won’t hear of it.
Don’t get me wrong, the Rwanda plan is a farce. If (a very big “if”) any are sent there the numbers will barely scratch the surface of the problem. It is not a deterrent because, as already demonstrated, the chances of any migrants being sent there are as close to zero as can be imagined. They have more chance of drowning making the crossing and since they are willing to take that risk, the tiny risk of being deported is but nothing.
As I keep repeating, once here the chances of any migrant being removed is absolutely minimal and even then there is nothing to prevent them repeating the process. The only way to deal with the problem is to forcibly prevent them from landing.