Would stay away from Heat Pumps unless you have underfloor heating and a thermal efficient house up to current building regs. Otherwise you would be wasting your money.
I have a number of properties with WB boilers (combi, condensing, etc) couple of properties have Glow worm. Cant fault either, certainly WB are the ones to go for, good service, back up and spares availability.
As far as condensing outlets freezing, i haven't had an issue as yet, just make sure the outlet pipe is well insulated / protected, obviously depends on where you live
Depending on storage the other alternative is a Biomass, but you need an area for the wood chips / pellets. Similar size storage area to a 1000 litre oil tank and reasonably close to the boiler.
Just noticed where you live....... forget Biomass, you may have a slight issue with a ready fuel supply