With little personal experience, here's my 0,02 €:
* France is first choice for clearing immigration, LFAT and LFAC most indicated as they see many Brits - most of them on a hamburger run but still
* Some Belgian fields are EBKT Kortrijk and EBOS Oostende, they are more expensive, especially EBOS
* I would not recommend Dutch fields, they are even more expensive. That is not their own fault, they are subject to many legal obligations. If you must, consider EHMZ Midden-Zeeland or EHTX Texel but I'm not sure the last one has customs.
* Fly high enough - FL50 or higher over Belgium will get you into controlled airspace. Especially recommended Mo-Fri when some of the military airspace may go active.
* Agree about Northern Germany, plenty of nice little grass fields, inexpensive, often a good restaurant too.