While you might think that banks would welcome people investing large amounts of money with them, many place limits upon how much can be used as a deposit to open an account. (With some accounts, that figure can be as low as £1,000). So your relative's range of choices isn't actually as wide as you might expect.
Further with the official Bank Rate currently at an all-time low of just 0.1%, no bank is able to offer particularly generous rates at the moment. If your relative is prepared to lock her money away, without access to it, for a full five years, the highest rate she can currently get (for such a large investment) is 1.55% with Shawbrook or UBL. If she can only lock her money away for a year, Shawbrook, at 1% is her best option. If she can only lock it away for 120 days, then Secure Trust, at 0.75%, offers the highest rate available to her. If she wants instant access to her money, Cynergy, with 0.5% is the best available offer at the moment.
With inflation (as measured by the Consumer Prices index) currently running at 2.1%, that means that there is NO bank that can offer an account where the value of her money won't actually DROP (in real terms) over the forthcoming years.
It's also worth remembering that just because a bank is 'Government-regulated', it won't help much if the bank you've invested £1m with goes bust. The limit for Government protection is £85,000 for each bank that a customer has an account with. So it would be wisest to spread the £1m across multiple banks.
Information sourced from here:
https://www.moneysavingexpert.com/savings/savings-accounts-best-interest/