There is never any guarantee that someone will win the top prize but they can guarantee the prize fund, which is the essence of the original question, because they have the reserves to do so.
EuroMillions smaller prizes do not affect higher category wins because each tier is limited to a set percentage of the total pot.
e.g.
Match 5 and 2 Lucky Stars (Highest prize) = 32% of the total prize fund
Match 4 and 1 Lucky Star (Middle-ish prize) = 1% of the total prize fund
Match 2 and 1 Lucky Star (Lowest prize) = 24% of the total prize fund
There is no "tenner" prize in EuroMillions. Last week's draw produced a lowest prize of £5.90. If there had been twice as many winners they would each have received £2.90 (prizes are rounded down to nearest 10p).
Likewise there is no guaranteed "tenner" prize in Lotto. The stated £10 is the maximum prize allowed; if the the prize fund is insufficient to give all Match 3 winners £10 the available fund will be split equally between all prize winners.