Well, you should first consult [http://www.reddit.com/r/ukpersonalfinance/wiki/lumpsuminvestment](I have £x, what should I do with it?), but here's what I would do:

Open an account with [Nutmeg](www.nutmeg.com), invest it all with a high risk (relatively - it means all the money's in shares but in reality the investments are in Index Funds. On the Nutmeg website you can play around with this). Along MrMoneyMustache principles, assuming this investment is for a long time (say, the rest of your life) you could reasonably expect a 7% increase per year. Taking average inflation into account, you would be able to remove 4% per year while keeping the invested amount continuing to grow, even relative to inflation.

So for example: Year 1: *£500,000 invested

Year 2: *Investment has grown 7% to £535,000 *Nutmeg charges 0.3%, reducing investment to £533,395 *You withdraw 4%, or £21,335.80, reducing investment to £512,059.20 *Inflation is assumed to average 2%, so even with the withdrawal your investment has still risen above inflation (2% inflation on £500,000 is £510,000).

Year 3: *Investment has grown 7% to £547,903.34 *Nutmeg charges 0.3%, reducing investment to £546,259.63 *You withdraw 4%, or £21,850.39 *Investment is now £524,409.24

From the sounds of it, your parents wouldn't be needing to withdraw the maximum 'safe' amount each year for the time being, so the investment can just sit compounding until needed. Example:

Year 2: *Investment has grown 7% to £535,000 *Nutmeg charges 0.3%, reducing investment to £533,395

Year 3: *Investment has grown 7% to £570,732.65 *Nutmeg charges 0.3%, reducing investment to £569,020.45

This is heavily based on Mr Money Mustache stuff, I would advise you to read his blog. I guess my real message is to plan for the long term and never withdraw more than you earn in interest (minus fees and inflation).