A good point. These lands did not belong to Germany prior to 1914, although they had been inhabited by Germans for many centuries. It is also of interest that German was the majority language in Prague until after 1860, and that it was the language of public announcements until about 1890. In older parts of the city the original German street names can sometimes still be seen, stencilled in attractive black Gothic script.
Does it make annexation of the region by Germany less appropriate to be supported by the UK?
No indeed. But I refer to recent political arrangements. Austria had never been owned by the German Empire, and neither had any part of Bohemia. The Treaty of Versailles forbade unification of Austria and Germany. Allowing Nazi Germany to annex everything in sight was not best way to amend that disastrous treaty.
I was intentionally avoiding reference to the remote past, having been rightly warned of its irrelevance. By German Empire I meant the pre-1914 Empire. Hitler did not claim all the territories of the HRE. He didn't even claim Czech Bohemia, though he later occupied it. He claimed the right to annex German inhabited lands. In fact he did not have this right under the prevailing international agreements then in force.
Have you thought this answer through? Hitler could have annexed the territory, and accorded to its Germanophone inhabitants any rights he saw fit to grant them. Meanwhile the Czechoslovak authorities could have retained the right of access to make suitable arrangements for the future security of their state. It could have gone on for as long as required, so long as the Czechoslovaks didn't interfere with the day to day administration of the area.