Supply Vs Demand and cadet labour are all valid arguments, but increasingly our poor situation in Europe in the face of increasing living costs is due to freedom of movement in Europe.
Wet lease/charter operators like Smartlynx, Avion, Enter Air, TravelService and previously Small Planet (combined 100 aircraft) pay a fixed circa €90k (that's Euros) or GBP75k for captains. They simply wouldn't be able to crew their flights out of UK, Germany and Scandinavia (where they do 90% of their business) if it wasn't for the thousands of Eastern European pilots from Bulgaria, Serbia, Romania, Latvia, Lithuania and Poland who take up those jobs. Those guys pay next to no tax and have their hotel accommodation is paid for wherever they operate in Europe. When they return home for their blocks off they usually retreat to a mansion in the countryside. The Western European pilots usually already live close to their bases so don't get an accommodation allowance and pay about 35% tax on the €90k / £75k. It's not a level playing field.
I don't see this changing after Brexit, if anything the CAA will give these operators the green light to continue operating as it would be seen as a Business to Business agreement between the tour operators or the airlines sub chartering work. We are a completely divided and de-unionised workforce thanks to Big Europe.