Great question. Alas, i wish i’d known the answer to this when i was in my 30’s and 40’s and was stupidly well paid.
The old addum of Money goes to Money is very true.
Open an ISA so you can benefit from Tax free growth of investments.
Open a SIPP so you can benefit from Tax free Growth and a sum which is not subject to IHT.
Open a GIA so you can make profits from money not in a Tax Wrapper as above.
Open a Savings account.
Open a bills account.
Keep a Current Accoubt and Credit Card.
Clear any debts, excluding mortgage.
Get 50% of the amount into the SIPP right away.
Fund the ISA Fully every year from the GIA.
Put about 10% maybe 20% into savings.
The balance into the GIA.
Monthly - service your current account from the savings account or GiA (depending if you have profits from the GIA) so you keep control of the flow and don’t go mad.
Monthly service the bills account from the saving account and keep a close eye on bills.
In terms of investments - steer clear of FA’s and be vary wary of IFA’s. Stick with Vangaurd who charge incredibly low fees. Focus on S&P 500 equities 80 to 100% and just buy and hold.
In essence your savings and the GIA are how you live day to day. GIA when you are in profits, ideally you want a 2 year buffer of cash, in your savings account to help ride out the markets bad times. Ie. You don’t sell anything in the GIA when times are bad.
Ultimately you are seeking to get everything into the tax free wrappers, so that you have more profits kept and not subject to income or lifetime allowance, hence it’s a 1 time sipp payment and nothing more. The isa has a 20k annual limit, use it every year. You can also consider junior Isa for the kids, and fill them annually too.
Ideally you should be able to make more from the ISA and the GIA than you pay towards the mortgage. If not, and you take a more cautious approach, clear the mortgage.
£1m is only life changing if you invest it and treat it as a 1 time event and not for splurging. Invested sensibly, with Vanguard or someone else low cost, it will make a huge difference to you and your kids life. Spend it quickly, and it will give you a lovely decade and it will be back to normal worrying about bills for everything.
To sum up, that £1m can reasonably give you an income of about £30k a year and leave a sizeable inheritance for the kids.