Loft insulation topped up to current recommended depths is usually the single best-value upgrade available, with a payback period of a few years and minimal disruption if the loft is already accessible.
Cavity wall insulation, where the wall type is suitable, is similarly cost-effective, though it needs a proper survey first — not every cavity wall is suitable, and getting it wrong can cause damp problems.
Window and door upgrades cost considerably more and take longer to pay back on energy savings alone, but they’re worth combining with other work you’re already doing (an extension, a renovation) rather than treated as a standalone project purely for insulation value.
