Curious, I just tried to reply to an identically worded post on the Microlight page on Facethingy just disappeared as I was replying to it.
Not disputing anything stated above (apart from RPS, which I find handy), but a useful rule of thumb is to use the airspace limits shown on the CAA half million chart. Look at the airspace top and bottom marked in your general vicinity and similar altitudes. If it's in feet, leave QNH on, if it's shown as a flight level set 1013 and use flight levels.
Not an official method, but works well enough and has never got me in trouble.
G