It's not bureaucracy that drives up costs, it's the total lack of federal funding. Most larger European airfields are private businesses. They pay corporation taxes, local taxes, they pay for runways, taxiways & hangars, all the staff at the airfield, inc ATC, work for the airfield. They pay for all of this by selling fuel & charging landing fees, hangarage, etc. Like most businesses they will shut down during those hours when there's not enough traffic to justify the costs of remaining open. I've never heard of a "government audit" of aircraft movements but, like any business, airfields will need to keep & file accounts. Some may also have planning ("zoning") requirements limiting annual movements.
Then there's a loooong tail of smaller airfields run as clubs or small businesses, each with their own modus operandi which in practice just means you ask first.
Flight plans are mandatory for flights that begin & end in different countries. Most of continental Europe (at least the western parts) are in a customs union. The UK isn't, so customs required to & from UK. In practice that means landing at a customs airfield and/or a few hours notice. From Europe to UK you can land in your back garden if you want, so long as you tell customs in advance when & where.
All but one country publishes an IAIP online, detailing all applicable regulations and larger airfield procedures. If, like Silvaire, you rarely venture outside your own personal goldfish bowl you might find this "procedurally complex" but in practice it's relatively simple. Thousands upon thousands of pilots do it regularly witthout a problem.