They of course want us to send SFI engineers with a risk of them charging a crazy amount instead of allowing us to pre-empt if the modem may be faulty by sending a test one out to a customer - like, proper ISP support should be like!
But if it is the modem that's faulty, the engineer will swap it FOC, it of course being Openreach property, and therefore their responsibility. It might also be noted that they will be carrying these as van stock, and thus save the ISP having to arrange to hold stock, delivery without it being damaged, (that's how they first split the atom, having Royal Mail deliver it ) making sure the EU is in to accept it, etc.
What if it isn't the modem ? D-side fault, faulty FTTC DSLAM port ? The SFI will do all this as well, FOC. All on the one visit. You send out a modem as you suggest, it's not the cause, you have wasted money, and the EU's still out of service. Is this what 'proper ISP support' would like ? I suspect not.
You bemoan the risk of having to pay for the cost of an SFI visit ? You wait until VDSL goes self install, THEN look at all the visits that end up to be EU internal wiring/set up. Just like good old DSL.