Similar to you, I am based in the UK, in the north. Specifically, near Leeds.
The proposed aluminium insulation foam might work. I don't know. Never tried it as there are no means of preventing the heat from building up if it gets too hot.
For not too much money you can create a temperature controlled setup. You need 3 pieces of kit:
- Inkbird temperature controller.
- 30cm Digital Probe thermometer (sometimes sold with the Inkbird, like this eBay listing).
Total cost for this will be around £40-50. Simply drill a hole in your bucket lid and add a gromet for a tight fit to get the temperature probe inside. It attaches to the temperature controller which is also connected to the brewbelt. You set it up to turn the heating element on (the brewbelt) when the temperature gets below a certain threshold, e.g. 20.5°C, and to turn off once it hits the desired temp, e.g. 21°C.
It also has a plug for a cooling element but given I'm in the north I've never had need of that (though I would if I wanted to do any lagering). If you have an old fridge to use you could ferment in there and this would turn the fridge on if the temp gets too hot.
I've used this setup with no issues for quite a while now. The brewing stays in the garage which is uninsulated.