Variable-rate certificate: is the monthly figure the bank quotes guaranteed?

No. A variable-rate certificate's rate is computed from a reference rate, such as one of the central bank's rates or the average interbank overnight rate, plus or minus the bank's margin. So any monthly figure you are given today is the expected return at today's reference rate, not a figure agreed for the whole term.

Hypothetical example per 100 pounds: a reference of 18% and a margin of 2% give a rate of 20%, with about 1.67 pounds paid each month. If the reference became 19% the payout would be 1.75; if it became 17%, about 1.58. This illustrates the relation; no forecast of where rates will go is implied.

A new rate applies after the current payout period's return is credited, and the margin follows the product's terms. The central bank sets its own rates; the certificate's rate is set by the bank.

If you want a figure agreed from the start, that is what a fixed rate offers; it is a trade-off between certainty and following the reference, not an advantage of one type over the other. Read in the key-facts statement: exactly which reference, the margin, when the rate changes, and whether there is a rate floor.