I penned a reply to Lilac Rain last night and decided I was probably not giving good advice and would leave it to others, but I find myself in agreement with the majority - that she should take steps to move out, rent her own place however small and animal-free (most are, but not all), in another area if costs near home are prohibitive.
Your life is yours, you only get one go at it. While I am all for helping parents in need, cross that bridge if or when it comes. You may be the unmarried daughter but you aren't the only one, it should not all fall to you (if it ever falls).
Despite lots of people saying they think your parents are lovely, well maybe that's part of their emotional blackmail tactic. My MIL had just one child, now my DH. He was engaged before he met me (he was 24) and they were buying a small house together. When the relationship broke up he told his mother that he was going to buy the house alone and move into it and she took an overdose, so he moved back home. That's emotional blackmail at its worse and I'm not saying your parents are like that, but it's more normal to encourage children to leave home and lead their own lives.
My own parents agreed between themselves that once I was married they would not get involved AT ALL if the marriage foundered unless due to abuse from my DH. I respected that. Extreme perhaps but at least I knew where I stood.
Get the flat. Take financial/benefits advice as others have said. Life is too short for "what-ifs" with your parents.