UK Northern Hemisphere - temperature fluctuations question

I have a quick beginner question.

I've been reading over a lot of posts on Shroomery and other sites, and I'm going with a BRF SGFC approach for my first grow of cubes.

I've downloaded videos as well from RoadRunner and everything all makes logical sense. Haven't quite gone so far as Agar and quality spores, I'm just using spore syringes from a vendor, especially while I'm learning.

One detail that crops up a lot is the ideal optimal temperature for cubes. Pretty much sounds like 84/85 degrees is optimal, yet people are suggesting as low as 70 degrees is acceptable - it's just going to slow your progress.

Here in the Northern Hemisphere (UK) we have some variable weather, within the same day/week it can fluctuate from 50 degrees to 70 degrees, often being at the warmer end during the day, and dropping hard at night.

I picked up a heating mat to try and maintain some consistency, but read a lot of negative feedback on this approach (plus there's the obvious dangers of fire and so on).

We don't have a heating system where you can put the grow near a constant heating source (ie we don't keep the heating on at night).

I've read about the TiT method, but I'm not sure how that helps keeping a constant temperature throughout consistent hours while you're asleep etc.

What's the issue surrounding temperature fluctuations - are the cubes going to die/be more prone to contamination with constant fluctuations in temperature, will they fail to actually grow if this is the case, OR, is it just a much slower process?

Help a noob out, who is doing plenty of online research (asking questions being one of them) - I'm not being lazy, I am reading constantly.

Thanks