Regarding interest (and saving for a house), read the MSE guide on lifetime ISA's (LISA). Govt matches 25% of what you put in, up to 4K, so effectively giving you 25% interest in 1st year. That obviously smooths out , over time so , after 6/7/8 years you would have earned that much in the normal highest earning savings accounts.
But you can put in another 4K the next year etc, and get 25% on those as well.
You need to open with at least £1 (age 18-40) but can shove any other money (up to 4K) in at any point, or not at all.