If the pressure has dropped significantly, that would indicate a leak I'm afraid, unless of course you've had a radiator off and forgotten to refill the system again. No idea on the re-ignite bit though, sorry. To re-pressurize, you need to get more water into the system at the pressure vessel. There should be a pressure guage close by so you can watch as you refill the system. Mine's in the false roof by the way but that doesn't mean that yours will be.
you re-fill the boiler via the filling loop which is normally a silver flexible pipe below the boiler with either 2 black levers on it or a screw slot type valve. you open both levers and you will hear the water start to flow into the boiler and watch the pressure gauge and then turn the levers back off when you get to 1.0 bar (when the boilers off i..e not running) you then need to vent the upstairs radiators of any air, if air does come out then you may need to add a little more water via the filling loop untl its up to 1.0 bar again.
if you then use the boiler and it drops down again over time then you need to call out an engineer as you either have a leak or the pressure relief valve is opning up to protect the boiler.
You may find the boiler is not working simply because the pressure has dropped, Follow guccimans very good instructions to refill it.
As for lighting it if i remember correct the Britony Combi was one of the first type to have an electronic ignition and therefore does not need lighting. You may however need to press the reset button in the middle of the control panel.