Not all equity release schemes have rolling interest - some offer a straightforward discounted sale price - for example a 50% share of a house valued at £200,000 for £75,000 so the remainder of the property still gains value, and can be willed in the usual way.
The most practical solution would be to buy a stake in the parental house - preferably with all siblings and future will contestants signing their agreement to reduce future conflict possibilites. This could be done by joint mortgage or extending the loan on another property - the issue becomes the potential risk to either or both properties should your circumstances change..
The seemingly obvious solution would be to buy the property outright, either on your own, or with any co-beneficiaries. Your parents could make a gift of the deposit, leaving you to find a mortgage only for the remainder, assuming you purchase on a buy to let, there is no reason on earth why you can't let to your parents, though you should not express that intention when looking for the loan perhaps. Your parents would have the cash lump sum to invest elsewhere, and you would receive a rental income to cover the purchase... As long as you can show the probable rental value is sufficiently high to support the borrowing amount, there is no compulsion to charge the full rent just a "reasonable" one.. so you could charge sufficient to cover the repayments each month.. offsetting income against interest only mortgage to avoid tax.
Come the day when your parents are no longer able to live in the house, or have passed on you would be liable for CGT on the profit, though you could technically switch residences long enough to declare the house as your primary residence and then sell in the usual way, and pay no tax.
Somewhat unorthodox, but thousands so it every year.