As an extra safeguard against mis-reading or mis-setting the altimter, most aviation GPS-based nav systems (such as Skydemon) have altimeter readouts based on sea level datum, equating (reasonably accurately) to local QNH.
I think I might be a bit careful using that method if planning to fly anywhere near the base of controlled airspace. That altitude measurement equates pretty much to a QNH of 1013. If the real QNH is 1003, that's 300 ft difference. For instance, if you were planning to fly at, say, 2300 ft through the Luton Stansted gap and used that readout........
If you are flying near controlled airspace: at best get a LARS service or, if you can't or don't want to do that, use a listening squawk. That way you should get a warning if you are about to infringe. I say SHOULD because it's still your responsibility not to.
