You could look into an air source heat pump if you have the space. We’re doing the same as you, rads in the house and UFH in an extension. After the £5k government grant the quotes for combi or ASHP were very similar.
9.5k inc VAT for a new boiler and pipework - though I'm pretty sure this was a "I don't want the job" quote
5k for ASHP, including the 5k gov rebate, no VAT
You could look into an air source heat pump if you have the space. We’re doing the same as you, rads in the house and UFH in an extension. After the £5k government grant the quotes for combi or ASHP were very similar.