I live in Michigan, about 30 miles from Lake Michigan, and one of our local TV stations gives an 8 day weather forecast. While the temperature they predict for day 8 may not hold true, it is very seldom off by that much - maybe 5 degrees either way. My guess is that the meteorological skills of weather folks in England are commensurate to those here. Point being, the scorching weather shouldn’t sneak up on the producers so much that they don’t have an inkling that filming day is going to be ungodly hot and ice cream cakes are doomed to fail. I don’t think it would be impossible (or even that difficult) for the show to come up with an alternate plan, either switching a particular bake or the week’s theme. For whatever reason, they simply have no desire to do so.