An ad says 'certificate at a high rate per year'. Will I get that rate every year?

Not necessarily. Some certificates are stepped or declining: their rate is highest in year one and then falls each year under a schedule published at the start. The ad usually shows only the year-one rate because it is the largest, so if you calculate as if it were flat for the whole term you will get a total larger than the real one. A hypothetical example per 100 pounds over 3 years, with made-up rates: if the schedule is 24%, then 18%, then 12%, the total is 54, but treating the 24% as flat gives 72. And if the income is paid monthly, it falls from year to year. That is not a flaw in the certificate, it is the shape of its schedule, but you need to see all of it before deciding. Note too that an ad is not proof of today's rate; rates change. Ask the bank for the rate schedule year by year and the return on your amount in each year, and check the rate type, the payout schedule and the break-penalty table in the key-facts statement.