OP you can't "protect the house", but what you can do is make sure your parents spend the money they've got on enjoying their final years. I don't mean something like equity release (or some dramatic change in lifestyle like gambling as mentioned in the charity advice link above which counts as depravation of assets!), but I do mean spending their pension on making their lives as enjoyable and easy as possible - do they have a cleaner, do they have a physio/personal trainer to help them keep fit and healthy, do they take taxis rather than waiting for the bus in the rain, do they treat you all to the odd family meal, do they keep the heating turned up. There is no point at this stage in life in making savings and cutting back. As some others have mentioned, do make sure they hold any savings in separate names. If it is more likely that one will need care than the other, then they can use mainly that person's savings for household extras. Unlike other benefits, the assets and income assessed are those of the person needing care, not the entire household, and if their partner is still alive the house won't be taken into account anyway. We know of couples living in £1mn+ houses where the husband is a higher rate tax payer, but only the wife's pittance of a pension is taken into account for her care home fees, so she contributes less than £100/month.
It is unfair that people who have frittered away all their money, then get care for free, but that's how our system works. There is also a lottery to the system - if you get dementia you pay, if you have a different long term health condition you may not. In such a lottery system, the govt should have stepped in to even things out across the population, but that means more tax and everyone thinks/hopes that they won't be the dementia case loser and so they don't want to pay more tax. Unfortunately, the new Labour govt instead of grasping this nettle and using their large majority to finally fix this, have kicked it into the long grass of another review when they could have just implemented the Dilnot review. Also though, if you're paying you have a choice, if the taxpayer is, you don't. And that isn't just important because of different standards of home but also location - they can place you in a care home anywhere in their region which may not be at all convenient for friends and relatives to visit.
There is a lesson here though for everyone. The time to downsize is in your 60s when you are fit enough and have the time to enjoy the capital you release. When you start getting doddery in your 70s/80s/90s, it's too late.
The other thing you can do before you need care is move to a home that is suitable for a live-in carer, or extend your home, or maintain your home in top notch condition so that it is easy to rent out and use the income to offset care home fees. We know of people who have done all these things and kept the family home safe for many years to be inherited by the children/grandchildren.
As more people see their assets disappearing into care home fees, there will be no incentive to save. This is the long term problem with the current system. Similarly the removal of tax breaks for pension savings will discourage people putting money into pensions, at the same time defined benefit pensions are disappearing (apart from in the public sector). In the future there won't be as many pensioners owning their own homes and enjoying large index-linked pensions. Then the chickens really will come home to roost.