Regards boiler,
With the intergas, my installer said I could run the combi side of it along with my hot water cylinder, meaning I could have mains hot water elsewhere in the house and have the hot water cylinder for my pumped shower.
In any case to answer your question, for boiler, the Atag is a reliable unit, but only installers can purchase it.
Vaillants I'm led to believe the combos aren't that reliable.
Worcester, great for parts availability and after sales, not as good as which magazine likes to make out.
Intergas, reliable and efficient, rarely go wrong, they are a rarity and most bodge it and scarper "specialists" don't like brands they don't know.
Baxi 600, good all rounder cheap.
Navien, good but unknown and pretty new to UK market
Vokera a bit like Baxi cheap all rounders fairly reliable.
Veissman, over priced, parts very expensive, hard to find, back up service is meh, like to use runner inside their boilers..
Main, essentially identical to the Baxi 600 just with less warranty, (owned by Baxi)
Biasi avoid
Ariston, own.atag, but surprisingly not as good as Atag, though they are generally cheaper.
Potterton are new build only now.
Daikin, rebadged intergas boilers except for their newest combi which is very good but few fitted in the UK
Can't think of any others.
Really speak to the bloke installing it and go with what he says.