You can use black ponds to absorb heat and put bubblewrap on top of the ponds for extra heat and insulation. You can greenhouse them too.
If you put some gravid females out in spring, then bring them in at autumn, you will probably get lots of young. The following year you put some of those young in the pond in spring and let them breed, and each year put young in. After 10 years you will have a cold water guppy strain.
We did it with rainbowfish here. I had a bunch of rainbowfish that were outdoors in ponds and I brought the adults in during autumn. There were a lot of babies in the ponds but most died in winter. The following spring they grew up and bred and within a few years I had fish that would live in water with a temperature of 5C in winter and 45C in summer.
In the UK it will take longer but it can be done during the warmer months.