Identify what you need
This guide from the Energy Saving Trust is a great place to start, it provides a host of recommendations for insulating your home.
It may be worth getting an Energy Performance Certificate (these are now required if you're selling, renting, or building a property). This certificate will include the typical energy usage/costs of your home as well as highlighting where improvements can be made.
This online tool from the UK Government will help you find an accredited person to undertake the work and provide the certificate.
Once you know where you can improve, it's time to get insulating! There are a few jobs you could tackle yourself if you're happy with DIY, others may require more specialist help.
When searching for suppliers, the Energy Saving Trust recommends looking for those that are registered with the National Insulation Association
This is where it may be useful to have the Energy Performance Certificate as it highlights both what needs to be done, and crucially: what you already have!
Why is this important?
Anything that can be done to improve energy efficiency, whether it's an A-rated fridge or an LED bulb, is a great step forward to a more carbon-free planet, and properly insulated homes are a great way to improve efficiency!
Properly insulating your home not only helps to reduce your energy bills and carbon footprint, it also makes renewable power options like solar panels much more attractive and viable for providing your power needs.