Unfortunately, you cannot check the expansion vessel's gas charge pressure with water in the boiler. The boiler must be drained first. The water pressure pushes the expansion vessel's diaphragm across, compressing the gas on the other side. When the two pressures are equal the diaphragm stops moving. So, the 1.5 bar that you measured is just the cold water pressure.
The minimum gas pressure for your (empty) expansion vessel is 0.5 bar (contact Baxi's Customer Services Department, on Tel:08706 049049, for details). Manufacturers pre-charge expansion vessels with Nitrogen (or sometimes Argon), because the use of an inert gas prevents corrosion. Dried compressed air can be used as a substitute when re-pressurising the vessel (the use of a foot pump being a last resort).
There's a pdf document about expansion vessels, which you may find useful,
here.
