Good for you. Most older U.K. houses are one brick thick (zero cavity) houses which are by their nature thermally dense. Thermally dense houses work by having the heating first heat the fabric of the house which then retains the warmth and radiates it back into the living space. It’s exactly the same principle as early man building structures from large rocks and having large fire pits within them - they didn’t choose large stones for shits and giggles. Heat exchangers don’t generate enough heat to heat the fabric of thermally dense buildings, nor are traditionally built U.K. houses readily suitable for additional insulation - you’re taking dry lining with insulated plasterboard to achieve ‘meh’ level benefits