but they do drive so why not go to the tip
For the amount of rubbish you describe, this is probably not an option as it sounds like there are many many car loads. Many tips only allow people who live in the same council area to use the tip. So unless the relatives live in the same council area, they might not be allowed to use the tip.
They might also struggle to use the tip in their own council, because many areas issue car passes and use number recognition to restrict the number of uses per year. You're only supposed to dispose of your own rubbish too, so technically, disposing of someone else's rubbish is not allowed.
It's a difficult situation. But do you know that the lady owned the house and not rented? If it was rented, obviously the landlord will want it back at some point, but if owned, you would have thought that the relatives would be interested in getting the house prepared for sale, including clearing all the rubbish, sooner rather than later.
If they genuinely don't have the money for skips/removal, which could be hundreds, if not thousands of pounds, I wonder if the council can clear the rubbish, and send a bill if there is one, to be paid when the house is sold?
Councils do have powers to order that derelict houses or excessively rubbish strewn gardens are made good, it's an environmental health function.