If you are looking to do your own heat-loss calculation to compare against 3rd party quotes or do some preliminary research. Then there are many options out there. I would recomend HeatPünk https://heatpunk.co.uk/ is actually a sureprisingly simple tool. Being fully web-based its easy to get setup with a free acount and setup your floor plans in a visual and intuitive manner. I currently have no point of reference to compare but will be looking to get professional heat-loss survey in future to compare. Noteable though is there is much debate over Air Changes/hr which on its own will dramatically affect your heat-loss estimates.

Heatpunk - the free, MCS-compliant heat loss and heat pump design software - has launched exciting new upgrades!

Its made by https://midsummerwholesale.co.uk/ which is also why they provde it free as if your interested you can transfer your system over to the Wholesalers main site for accurate pricing quote!