Important questions are – what is your appetite for risk, how quickly do you want to invest, and when do you think you’ll want to access the funds?
Assuming the answers are (a) medium/low; (b) not fussed; and (c) age 65.
If it were me, based on the above, I would open a stocks and shares ISA (with Halifax, who have the lowest annual management charge of £12.50/yr).
Within that, I’d then set up an autoinvestment of £1666.66 / month (which will neatly put in £20k/yr – the max ISA limt) to buy Vanguard Target Retirement 2035: https://www.vanguardinvestor.co.uk/investments/vanguard-target-retirement-2035-fund-accumulation-shares
With the uninvested cash, I would put some in Premium Bonds and – if you can be bothered to fiddle – set up monthly savings accounts with (a) Nationwide (where you can 5% on £250/month); (b) First Direct (5% on £300 a month); (c) Virgin (in branch you can get 3% on £300/month).
That’ll be £400 please 😉