[kezo]

Turning a modern boiler downd to 60c will have no adverse effect on heating your home and a boiler will be at it most efficient. It will condense as the name suggests condensing boiler and be stated in the boiler manual. for my baxi it is 60-65. The link below is for Vaillant, but proves what use to be 80 flow/60 return rule is out of the window with a modern boiler.

https://www.vaillant.co.uk/homeowners/advice-and-knowledge/combiboilertemperaturesetting-2156808.html#:~:text=For%20a%20modern%20combi%20boiler%2C%20the%20radiator%20heating,and%20be%20at%20its%20optimum%20efficiency%20and%20performance.

My house is a 4 bed dormer style buit in 1963. Other than loft and cavity wall insulation the dormer insulation remains how it was built. With my boiler set temp at 60c it heats the home easily up to 25c if I wished.