Boilers in the UK are usually mounted on the inside of an exterior wall to provide for venting exhaust gases outside. They may provide hot water on-demand without a tank (combi boiler) or fill a hot water cylinder on a schedule (system boiler) depending on type. Heating is typically provided by the boiler pumping hot water through a circuit of wall-mounted radiators regulated by TRVs (Thermostatic Radiator Valves).
Boilers are definitely much more common in the UK than US. I believe New England is one of the few areas of the US that has a relatively high percentage of homes with hydronic+boiler heating setups. In most other parts of the U.S., many people probably have no idea what a boiler even is.