What's the best way to insulate a Victorian terraced house?

Victorian terraced houses are tricky because you want to preserve their character while actually making them livable in terms of energy efficiency. The main issue is that these places were built without any insulation whatsoever, so you're basically starting from scratch.

For the walls, you've got a few options depending on your budget and how much disruption you can tolerate. Cavity wall insulation is the least invasive if your walls have a cavity (though many Victorian terraces don't), but honestly most of these houses have solid brick walls. In that case you're looking at either external wall insulation, which involves cladding the outside and can change the appearance (sometimes worth it, sometimes not), or internal wall insulation, which eats into your floor space but keeps the original façade intact. Internal is generally cheaper and less hassle with planning permission issues, but it does reduce room sizes slightly. The loft is where you should start though - that's the easiest win. Get that properly insulated (at least 250-300mm of mineral wool or similar) and you'll notice a real difference.

Don't neglect the floors and basement if you have one. Suspended wooden floors are common in Victorians and they're basically like having a sieve - you get cold air coming up constantly. Underfloor insulation or even just blocking the air vents (partially - you need some ventilation) helps loads. Windows are obviously important too; secondary glazing can work well for keeping the period frames while improving thermal performance. The key is taking it step by step, prioritizing the biggest heat losses first. I've got a colleague who's obsessed with this stuff so I keep hearing about her Victorian renovation (she got a dog specifically to keep warm lol), and she swears the loft insulation alone was the best return on investment.